kmod-dma-buf: fix build with external kernel

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>

package/kernel/linux/modules/other.mk

@@ -129,7 +129,11 @@ define KernelPackage/dma-buf
   TITLE:=DMA shared buffer support
   HIDDEN:=1
   KCONFIG:=CONFIG_DMA_SHARED_BUFFER
-  FILES:=$(LINUX_DIR)/drivers/dma-buf/dma-shared-buffer.ko
+  ifeq ($(strip $(CONFIG_EXTERNAL_KERNEL_TREE)),"")
+    ifeq ($(strip $(CONFIG_KERNEL_GIT_CLONE_URI)),"")
+      FILES:=$(LINUX_DIR)/drivers/dma-buf/dma-shared-buffer.ko
+    endif
+  endif
   AUTOLOAD:=$(call AutoLoad,20,dma-shared-buffer)
 endef
 $(eval $(call KernelPackage,dma-buf))