[PATCH 3/3] selftests/gpio: unset OUTPUT for build tools/gpio

when we execute 'make' to build selftests, the TOP Makefile build gpio like: selftests$ make ARCH= CROSS_COMPILE= OUTPUT=/home/lizj/linux/tools/testing/selftests/gpio -C gpio ... make[2]: Leaving directory '/home/lizj/linux/tools/gpio' gcc -O2 -g -std=gnu99 -Wall

[PATCH 3/3] selftests/gpio: unset OUTPUT for build tools/gpio

when we execute 'make' to build selftests, the TOP Makefile build gpio like: selftests$ make ARCH= CROSS_COMPILE= OUTPUT=/home/lizj/linux/tools/testing/selftests/gpio -C gpio ... make[2]: Leaving directory '/home/lizj/linux/tools/gpio' gcc -O2 -g -std=gnu99 -Wall