In hack/904-debloat_dma_buf.patch, DMA_SHARED_BUFFER is changed from
bool to tristate. As this patch is not applied to external kernel
sources, build fails if kmod-dma-buf is enabled. Fix this by only
including the module file if CONFIG_EXTERNAL_KERNEL_TREE and
CONFIG_KERNEL_GIT_CLONE_URI are not enabled.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>