ssb: SSB_DRIVER_GPIO should depend on GPIOLIB instead of selecting it

Commit ec43b08b5733494ad88aa618ecdf534320dd8207 ("ssb: add GPIO driver")
added SSB_DRIVER_GPIO, which unconditionally selects GPIOLIB, causing
a Kconfig warning:

warning: (ARCH_REQUIRE_GPIOLIB && SSB_DRIVER_GPIO && BCMA_DRIVER_GPIO && MFD_TC6393XB && FB_VIA) selects GPIOLIB which has unmet direct dependencies (ARCH_WANT_OPTIONAL_GPIOLIB || ARCH_REQUIRE_GPIOLIB)

and build failure for m68k/allmodconfig:

In file included from drivers/ssb/ssb_private.h:5,
                 from drivers/ssb/main.c:12:
include/linux/ssb/ssb.h:440: error: field ‘gpio’ has incomplete type
make[4]: *** [drivers/ssb/main.o] Error 1
make[3]: *** [drivers/ssb/] Error 2

Turn the select into a dependency to fix this.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>