drivers/apds99xx: Fix includes

The driver does not depend on or use the xtimer module, it should therefore
not include `xtimer.h`. Same for the test, which additionally missed an
include of `mutex.h` (previously implicitly included through `xtimer.h`).

drivers/apds99xx/apds99xx.c

@@ -23,7 +23,6 @@
 
 #include "irq.h"
 #include "log.h"
-#include "xtimer.h"
 
 #define ENABLE_DEBUG    (0)
 #include "debug.h"

tests/driver_apds99xx_full/main.c

@@ -35,9 +35,9 @@
 
 #include <stdio.h>
 
+#include "mutex.h"
 #include "thread.h"
 #include "thread_flags.h"
-#include "xtimer.h"
 
 #include "apds99xx.h"
 #include "apds99xx_params.h"