net: dsa: mt7530: don't build GPIO support if !GPIOLIB

net: dsa: mt7530: don't build GPIO support if !GPIOLIB

The new GPIO support may be optional at runtime, but it requires
building against gpiolib:

ERROR: modpost: "gpiochip_get_data" [drivers/net/dsa/mt7530.ko]
undefined!
ERROR: modpost: "devm_gpiochip_add_data_with_key"
[drivers/net/dsa/mt7530.ko] undefined!

Add #ifdef to exclude GPIO support if GPIOLIB is not enabled.

Fixes: 429a0edeefd8 ("net: dsa: mt7530: MT7530 optional GPIO support")
Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: DENG Qingfang <dqfext@gmail.com>
Link: https://lore.kernel.org/r/20210226063226.8474-1-dqfext@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>