examples/*: add missing include of assert.h

examples/dtls-echo/dtls-client.c

@@ -21,6 +21,7 @@
  * @}
  */
 
+#include <assert.h>
 #include <stdio.h>
 #include <inttypes.h>
 

examples/dtls-echo/dtls-server.c

@@ -22,6 +22,7 @@
  * @}
  */
 
+#include <assert.h>
 #include <stdio.h>
 #include <inttypes.h>
 

examples/nimble_gatt/main.c

@@ -31,6 +31,7 @@
  * @}
  */
 
+#include <assert.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>