selftests: dmabuf-heaps: Fix incorrect kernel headers search path

Use $(KHDR_INCLUDES) as lookup path for kernel headers. This prevents
building against kernel headers from the build environment in scenarios
where kernel headers are installed into a specific output directory
(O=...).
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: linux-kselftest@vger.kernel.org
Cc: Ingo Molnar <mingo@redhat.com>
Cc: <stable@vger.kernel.org>  # 5.18+
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/dmabuf-heaps/Makefile

 # SPDX-License-Identifier: GPL-2.0
-CFLAGS += -static -O3 -Wl,-no-as-needed -Wall
+CFLAGS += -static -O3 -Wl,-no-as-needed -Wall $(KHDR_INCLUDES)
 
 TEST_GEN_PROGS = dmabuf-heap
 

tools/testing/selftests/dmabuf-heaps/dmabuf-heap.c

@@ -13,10 +13,9 @@
 #include <sys/types.h>
 
 #include <linux/dma-buf.h>
+#include <linux/dma-heap.h>
 #include <drm/drm.h>
 
-#include "../../../../include/uapi/linux/dma-heap.h"
-
 #define DEVPATH "/dev/dma_heap"
 
 static int check_vgem(int fd)