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 <[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/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