egl: Fix implicit declaration of ffs

Found when building for Android in C99 mode. Include bitscan.h to ensure ffs is
available.

Fixes: 7b4ed2b5 ("egl: Convert configs to use shifts and sizes instead of masks")

Signed-off-by: Kevin Strasser <kevin.strasser@intel.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>

src/egl/drivers/dri2/egl_dri2.c

@@ -65,6 +65,7 @@
 #include "util/u_atomic.h"
 #include "util/u_vector.h"
 #include "mapi/glapi/glapi.h"
+#include "util/bitscan.h"
 
 /* Additional definitions not yet in the drm_fourcc.h.
  */