mesa_dri2: Add missing include of config.h to define _GNU_SOURCE

Fix build with -Wimplicit-function-declaration while secure_getenv() is
guarded by __USE_GNU.
Reviewed-by: Aaron Plattner <aplattner@nvidia.com>
Tested-by: Stefan Dirsch <sndirsch@suse.de>

src/mesa_dri2.c

@@ -33,6 +33,9 @@
  *   and José Hiram Soltren (jsoltren@nvidia.com)
  */
 
+#ifdef HAVE_CONFIG_H
+#include "config.h"
+#endif
 
 #define NEED_REPLIES
 #include <X11/Xlibint.h>