selftests: filesystems: 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/filesystems/Makefile

···
 # SPDX-License-Identifier: GPL-2.0
 
-CFLAGS += -I../../../../usr/include/
+CFLAGS += $(KHDR_INCLUDES)
 TEST_GEN_PROGS := devpts_pts
 TEST_GEN_PROGS_EXTENDED := dnotify_test
 

tools/testing/selftests/filesystems/binderfs/Makefile

···
 # SPDX-License-Identifier: GPL-2.0
 
-CFLAGS += -I../../../../../usr/include/ -pthread
+CFLAGS += $(KHDR_INCLUDES) -pthread
 TEST_GEN_PROGS := binderfs_test
 
 binderfs_test: binderfs_test.c ../../kselftest.h ../../kselftest_harness.h

tools/testing/selftests/filesystems/epoll/Makefile

···
 # SPDX-License-Identifier: GPL-2.0
 
-CFLAGS += -I../../../../../usr/include/
+CFLAGS += $(KHDR_INCLUDES)
 LDLIBS += -lpthread
 TEST_GEN_PROGS := epoll_wakeup_test