compositor: export weston_buffer_from_resource()

This is needed to allow shell plugins to implement their own
screenshooter.
diff --git a/src/compositor.c b/src/compositor.c
index 016f514..47a806e 100644
--- a/src/compositor.c
+++ b/src/compositor.c
@@ -1442,7 +1442,7 @@
 	free(buffer);
 }
 
-struct weston_buffer *
+WL_EXPORT struct weston_buffer *
 weston_buffer_from_resource(struct wl_resource *resource)
 {
 	struct weston_buffer *buffer;