selftests: gpio: Fix OUTPUT directory in Makefile

selftests: gpio: Fix OUTPUT directory in Makefile

When simply running `make' from the selftests top dir, this
error shows up:

  cc -O2 -g -std=gnu99 -Wall -I../../../../usr/include/ -I/usr/include/libmount -I/usr/include/blkid -I/usr/include/uuid    gpio-mockup-chardev.c ../../../gpio/gpio-utils.o  -lmount -o gpio-mockup-chardev
  cc: error: ../../../gpio/gpio-utils.o: No such file or directory
  <builtin>: recipe for target 'gpio-mockup-chardev' failed
  make[1]: *** [gpio-mockup-chardev] Error 1

because the output directory is set to "selftests/gpio" and
all binaries built from ../../../gpio/ end up there. In fact,
they appear as, exempli gratia:
* gpiogpio-event-mon
* gpiogpio-hammer
* gpioinclude/
* gpiolsgpio
which is wrong, as it's missing a directory separator
somewhere.

This patch sets straight the output directory when building
../../../gpio/ so that binaries don't cross paths.

Signed-off-by: Daniel Díaz <daniel.diaz@linaro.org>
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>