selftests: gpio: fix gpio compiling error

[ Upstream commit 92a59d7f38 ]

The gpio selftests build against the system includes rather than the
headers from the linux tree.  This results in the compile failing if
the system includes are outdated.

Prefer the headers from the linux tree, as per other selftests.

Fixes: 8bc395a6a2 ("selftests: gpio: rework and simplify test implementation")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
[Kent: reworded commit comment and added Fixes:]
Signed-off-by: Kent Gibson <warthog618@gmail.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Signed-off-by: Sasha Levin <sashal@kernel.org>

tools/testing/selftests/gpio/Makefile

@@ -3,5 +3,6 @@
 TEST_PROGS := gpio-mockup.sh
 TEST_FILES := gpio-mockup-sysfs.sh
 TEST_GEN_PROGS_EXTENDED := gpio-mockup-cdev
+CFLAGS += -I../../../../usr/include
 
 include ../lib.mk