examples/*: add explicit include of timex.h

examples/dtls-echo/dtls-client.c

@@ -24,6 +24,7 @@
 #include <stdio.h>
 #include <inttypes.h>
 
+#include "timex.h"
 #include "net/sock/udp.h"
 #include "tinydtls_keys.h"
 

examples/dtls-echo/dtls-server.c

@@ -27,6 +27,7 @@
 
 #include "net/sock/udp.h"
 #include "msg.h"
+#include "timex.h"
 #include "tinydtls_keys.h"
 
 /* TinyDTLS */

examples/dtls-sock/dtls-server.c

@@ -23,6 +23,7 @@
 #include "net/credman.h"
 #include "msg.h"
 #include "thread.h"
+#include "timex.h"
 
 #include "tinydtls_keys.h"
 

examples/nimble_heart_rate_sensor/main.c

@@ -25,6 +25,7 @@
 #include "event/timeout.h"
 #include "nimble_riot.h"
 #include "net/bluetil/ad.h"
+#include "timex.h"
 
 #include "host/ble_hs.h"
 #include "host/ble_gatt.h"