selftests: ipc: 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/ipc/Makefile

@@ -10,7 +10,7 @@ ifeq ($(ARCH),x86_64)
 	CFLAGS := -DCONFIG_X86_64 -D__x86_64__
 endif
 
-CFLAGS += -I../../../../usr/include/
+CFLAGS += $(KHDR_INCLUDES)
 
 TEST_GEN_PROGS := msgque