selftests: futex: 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/futex/functional/Makefile

 # SPDX-License-Identifier: GPL-2.0
-INCLUDES := -I../include -I../../ -I../../../../../usr/include/
+INCLUDES := -I../include -I../../ $(KHDR_INCLUDES)
 CFLAGS := $(CFLAGS) -g -O2 -Wall -D_GNU_SOURCE -pthread $(INCLUDES) $(KHDR_INCLUDES)
 LDLIBS := -lpthread -lrt