fix for "dma-buf: move dma-buf symbols into the DMA_BUF module namespace"

Adds the proper MODULE_IMPORT_NS(DMA_BUF) line to the file to get it to
build properly.
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Link: https://lore.kernel.org/r/20211027154843.622961fd@canb.auug.org.auSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/gpu/drm/drm_gem_shmem_helper.c

@@ -17,6 +17,8 @@
 #include <drm/drm_prime.h>
 #include <drm/drm_print.h>
 
+MODULE_IMPORT_NS(DMA_BUF);
+
 /**
  * DOC: overview
  *