mesa: Mark default case unreachable to silence warning.

Warned about 'coord' being undefined in the default case, which is
unreachable.

Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/mesa/drivers/common/meta.c

@@ -2614,6 +2614,7 @@ _mesa_meta_setup_texture_coords(GLenum faceTarget,
             break;
          default:
             assert(0);
+            unreachable();
          }
 
          coord[3] = (float) (slice / 6);