selftests: futex: Add the uapi headers include variable

Out of tree build of this test fails if relative path of the output
directory is specified. KBUILD_OUTPUT also doesn't point to the correct
directory when relative path is used. Thus out of tree builds fail.
Remove the un-needed include paths and use KHDR_INCLUDES to correctly
reach the headers.
Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
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/ \
-	-I$(KBUILD_OUTPUT)/kselftest/usr/include
-CFLAGS := $(CFLAGS) -g -O2 -Wall -D_GNU_SOURCE -pthread $(INCLUDES)
+INCLUDES := -I../include -I../../ -I../../../../../usr/include/
+CFLAGS := $(CFLAGS) -g -O2 -Wall -D_GNU_SOURCE -pthread $(INCLUDES) $(KHDR_INCLUDES)
 LDLIBS := -lpthread -lrt
 
 HEADERS := \