shell: Fix warning visible with latest Newlib

Works also with previous Newlib versions.

cpukit/libmisc/shell/main_date.c

@@ -16,10 +16,12 @@
 #include "config.h"
 #endif
 
+#define _GNU_SOURCE
 #include <stdio.h>
 #include <unistd.h>
 #include <string.h>
 #include <errno.h>
+#include <time.h>
 
 #include <rtems.h>
 #include <rtems/shell.h>