tools: gpio: Use !building_out_of_srctree to determine srctree

make TARGETS=gpio kselftest fails with:

Makefile:23: tools/build/Makefile.include: No such file or directory

When the gpio tool make is invoked from tools Makefile, srctree is
cleared and the current logic check for srctree equals to empty
string to determine srctree location from CURDIR.

When the build in invoked from selftests/gpio Makefile, the srctree
is set to "." and the same logic used for srctree equals to empty is
needed to determine srctree.

Check building_out_of_srctree undefined as the condition for both
cases to fix "make TARGETS=gpio kselftest" build failure.

Cc: stable@vger.kernel.org
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>