userspace/tools: warning fix implicit declatarion of usleep in wr_mon

Signed-off-by: Adam Wujek <adam.wujek@cern.ch>

userspace/tools/wr_mon.c

@@ -2,6 +2,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <getopt.h>
+#include <unistd.h>
 
 #include <libwr/shmem.h>
 #include <libwr/hal_shmem.h>