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 <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Cc: [e-mail]
Cc: Ingo Molnar <[e-mail]>
Cc: <[e-mail]> # 5.18+
Signed-off-by: Shuah Khan <[e-mail]>

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

···
 #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"