demo: Add headers to overview.c (#684)

This adds the required C headers directly to the top of the overview.c demo so that they don't need to be added in the renders themselves.

demo/common/overview.c

@@ -1,4 +1,5 @@
-#include <time.h>
+#include <limits.h> /* INT_MAX */
+#include <time.h> /* struct tm, localtime */
 
 static int
 overview(struct nk_context *ctx)