selftests/gpio: Move include of lib.mk up

Move the include of lib.mk up so that in a subsequent patch we can use
OUTPUT, which is initialised by lib.mk, in the definition of the GPIO
variables.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>