staging: mt7621-gpio: mt7621: make symbol gc_map static

Fixes the following sparse warning:

drivers/staging/mt7621-gpio/gpio-mt7621.c:47:3: warning:
 symbol 'gc_map' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>