selftests/gpio: Use TEST_GEN_PROGS_EXTENDED

Use TEST_GEN_PROGS_EXTENDED rather than TEST_PROGS_EXTENDED.

That tells the lib.mk logic that the files it references are to be
generated by the Makefile.

Having done that we don't need to override the all rule.

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