selftests: sync: 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+
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/sync/Makefile

 # SPDX-License-Identifier: GPL-2.0
 CFLAGS += -O2 -g -std=gnu89 -pthread -Wall -Wextra
-CFLAGS += -I../../../../usr/include/
+CFLAGS += $(KHDR_INCLUDES)
 LDFLAGS += -pthread
 
 .PHONY: all clean