selftests: dma-buf: Adding kernel config fragment CONFIG_UDMABUF=y

The test case drivers/dma-buf/udmabuf need this kernel config enabled

CONFIG_UDMABUF=y

Signed-off-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>