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the system includes are outdated.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: Nkernel test robot <lkp@intel.com>
Signed-off-by: NLi Zhijian <lizhijian@cn.fujitsu.com>
[Kent: reworded commit comment and added Fixes:]
Signed-off-by: NKent Gibson <warthog618@gmail.com>
Signed-off-by: NBartosz 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