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.au


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>