minigbm: add gbm_bo_get_fd_for_plane

One more piece in aligning with Mesa GBM ..

BUG=b:145747113
TEST=compile

Change-Id: Ic9d5dad5705feccd7005f40370f59d0339fe8bfc
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/minigbm/+/2803580
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Commit-Queue: Gurchetan Singh <gurchetansingh@chromium.org>
diff --git a/gbm.h b/gbm.h
index 5ccd520..10c172e 100644
--- a/gbm.h
+++ b/gbm.h
@@ -429,6 +429,9 @@
 gbm_bo_get_handle_for_plane(struct gbm_bo *bo, size_t plane);
 
 int
+gbm_bo_get_fd_for_plane(struct gbm_bo *bo, int plane);
+
+int
 gbm_bo_write(struct gbm_bo *bo, const void *buf, size_t count);
 
 void