WINGs: Added include for header defining the funcion provided by the file

This allows the compiler to warn if the definition in the file is
no in line with what is exposed to the users of the function through
the header definition.

WINGs/usleep.c

@@ -2,6 +2,7 @@
 #include <errno.h>
 #include <time.h>
 
+#include "WUtil.h"
 #include "wconfig.h"
 
 void wusleep(unsigned int usec)