gallium: Silence unused variable warning

Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Signed-off-by: Elie Tournier <elie.tournier@collabora.com>
Signed-off-by: Jakob Bornecrantz <jakob@collabora.com>

src/gallium/auxiliary/util/u_surface.c

@@ -254,7 +254,8 @@ util_resource_copy_region(struct pipe_context *pipe,
    struct pipe_transfer *src_trans, *dst_trans;
    uint8_t *dst_map;
    const uint8_t *src_map;
-   enum pipe_format src_format, dst_format;
+   MAYBE_UNUSED enum pipe_format src_format;
+   enum pipe_format dst_format;
    struct pipe_box dst_box;
 
    assert(src && dst);