ui-repolist: avoid build warning for strcasestr(3)

The non-standard function strcasestr is only defined if _GNU_SOURCE has
also been defined.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

ui-repolist.c

@@ -6,6 +6,10 @@
  *   (see COPYING for full license text)
  */
 
+/* This is needed for strcasestr to be defined by <string.h> */
+#define _GNU_SOURCE 1
+#include <string.h>
+
 #include <time.h>
 
 #include "cgit.h"