gnrc_pktbuf_static: fix alignment issue / leaks

This fixes an alignment issue I encountered in the static version of
the packet buffer.

The bug is caused by a race-condition where a certain order of
operations leads to a chunk being released according to the
byte-alignment of the platform, but overlapping potential space for
a future `_unused_t` struct e.g. (x mark allocated regions):

                    Future leak of size sizeof(_unused_t)       Time
                    v                                            |
    +------------+-----+--------------------+                    |
    |xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx|                    +
    +------------+-----+--------------------+                    |
                                                                 |
    +------------+--+--+--------------------+                    |
    |               |xxxxxxxxxxxxxxxxxxxxxxx|                    +
    +------------+--+--+--------------------+                    |
                                                                 |
    +-----+------+--+--+--------------------+                    |
    |xxxxx|         |xxxxxxxxxxxxxxxxxxxxxxx|                    +
    +-----+------+--+--+--------------------+                    |
                                                                 |
    +-----+------+-----+---------+----------+                    |
    |xxxxx|                      |xxxxxxxxxx|                    +
    +-----+------+-----+---------+----------+                    |
                                                                 |
    +-----+------+-----+--------------------+                    |
    |xxxxx|      |xxxxxxxxxxxxxxxxxxxxxxxxxx|                    +
    +-----+------+-----+--------------------+                    |
                                                                 |
    +------------+-----+--------------------+                    |
    |xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx|                    +
    +------------+-----+--------------------+                    |
                                                                 |
    +------------+-----+--------------------+                    |
    |xxxxxxxxxxxxxxxxxx|                    |                    +
    +------------+-----+--------------------+                    |
                                                                 |
    +------------+-----+--------------------+                    |
    |            |xxxxx|                    |                    +
    +------------+-----+--------------------+                    |
                                                                 v

Sadly, I wasn't able to create a reproducable unittest that show-cases
this corner-case, since I don't understand the order of operations that
cause this one 100%, but the bug is reproducable (but also not
reliably) by sending large (i.e. fragmented) packets to a 6Lo-enabled
host from more than 1 host simultaneously (use `gnrc_pktbuf_cmd` to
check).

By making the size of `_unused_t` the only condition for alignment,
this bug is fixed.

sys/net/gnrc/pktbuf_static/gnrc_pktbuf_static.c

@@ -33,7 +33,7 @@
 #define ENABLE_DEBUG (0)
 #include "debug.h"
 
-#define _ALIGNMENT_MASK    (sizeof(void *) - 1)
+#define _ALIGNMENT_MASK    (sizeof(_unused_t) - 1)
 
 typedef struct _unused {
     struct _unused *next;