[PATCH v2] selftests/gpio: Fix gpio compiling error

[Li Zhijian <lizhijian@xxxxxxxxxxxxxx>]

[root@iaas-rpma gpio]# make
gcc     gpio-mockup-cdev.c  -o /home/lizhijian/linux/tools/testing/selftests/gpio/gpio-mockup-cdev
gpio-mockup-cdev.c: In function ‘request_line_v2’:
gpio-mockup-cdev.c:24:30: error: storage size of ‘req’ isn’t known
   24 |  struct gpio_v2_line_request req;
      |                              ^~~
gpio-mockup-cdev.c:32:14: error: ‘GPIO_V2_LINE_FLAG_OUTPUT’ undeclared (first use in this function); did you mean ‘GPIOLINE_FLAG_IS_OUT’?
   32 |  if (flags & GPIO_V2_LINE_FLAG_OUTPUT) {
      |              ^~~~~~~~~~~~~~~~~~~~~~~~

gpio-mockup-cdev.c includes <linux/gpio.h> which could be provided by
kernel-headers package, and where it's expected to declare
GPIO_V2_LINE_FLAG_OUTPUT. However distros or developers will not always
install the same kernel-header as we are compiling.

So we can tell compiler to search headers from linux tree simply like others,
such as sched.

CC: Philip Li <philip.li@xxxxxxxxx>
Reported-by: kernel test robot <lkp@xxxxxxxxx>
Signed-off-by: Li Zhijian <lizhijian@xxxxxxxxxxxxxx>

---
V2: add more details about the fix
---
 tools/testing/selftests/gpio/Makefile | 1 +
 1 file changed, 1 insertion(+)

diff --git a/tools/testing/selftests/gpio/Makefile b/tools/testing/selftests/gpio/Makefile
index 39f2bbe8dd3d..42ea7d2aa844 100644
--- a/tools/testing/selftests/gpio/Makefile
+++ b/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
-- 
2.31.1