selftests: build and run gpio when output directory is the src dir

Build and run gpio when output directory is the src dir.  gpio has
dependency on tools/gpio and builds tools/gpio objects in the src
directory in all cases making the src repo dirty even when object
relocation is specified.

This fixes the following commands from generating gpio objects in
the source repository:

make O=dir kselftest
export KBUILD_OUTPUT=dir; make kselftest
make O=dir -C tools/testing/selftests
expoert KBUILD_OUTPUT=dir; make -C tools/testing/selftests

The following commands still build gpio objects in the source repo
(gpio Makefile needs to fixed):
make O=dir kselftest TARGETS="gpio"
export KBUILD_OUTPUT=dir; make kselftest TARGETS="gpio"
make O=dir -C tools/testing/selftests TARGETS="gpio"
expoert KBUILD_OUTPUT=dir; make -C tools/testing/selftests TARGETS="gpio"

Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>