Fix build issue when gbm backend is built without dri support.

re-use patch by Spenser
http://lists.busybox.net/pipermail/buildroot/2013-August/076839.html

Signed-off-by: Spenser Gilliland <spenser at gillilanding.com>
Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>

diff -uNr Mesa-10.0.5.org/src/gbm/main/backend.c Mesa-10.0.5/src/gbm/main/backend.c
--- Mesa-10.0.5.org/src/gbm/main/backend.c	2013-12-17 00:19:27.000000000 +0100
+++ Mesa-10.0.5/src/gbm/main/backend.c	2014-04-29 07:12:38.175264298 +0200
@@ -36,7 +36,9 @@
 
 #define ARRAY_SIZE(a) (sizeof(a)/sizeof((a)[0]))
 
+#ifdef HAVE_DRI
 extern const struct gbm_backend gbm_dri_backend;
+#endif
 
 struct backend_desc {
    const char *name;
@@ -44,7 +46,9 @@
 };
 
 static const struct backend_desc backends[] = {
+#ifdef HAVE_DRI
    { "gbm_dri.so", &gbm_dri_backend },
+#endif
    { "gbm_gallium_drm.so", NULL },
 };