selftests: gpio: fix gpio compiling error

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the system includes are outdated.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: 8bc395a6 ("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>

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