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>

drivers/staging/mt7621-gpio/gpio-mt7621.c

@@ -38,7 +38,7 @@ static void __iomem *mediatek_gpio_membase;
 static int mediatek_gpio_irq;
 static struct irq_domain *mediatek_gpio_irq_domain;
 
-struct mtk_gc {
+static struct mtk_gc {
 	struct gpio_chip chip;
 	spinlock_t lock;
 	int bank;