MAINTAINERS: Add a missing file to the INTEL GPIO section

When gpio-tangier was split the new born headers had been missed
in the MAINTAINERS. Add it there.

Fixes: d2c19e89 ("gpio: tangier: Introduce Intel Tangier GPIO driver")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>

MAINTAINERS

@@ -10646,6 +10646,7 @@ F:	drivers/gpio/gpio-pch.c
 F:	drivers/gpio/gpio-sch.c
 F:	drivers/gpio/gpio-sodaville.c
 F:	drivers/gpio/gpio-tangier.c
+F:	drivers/gpio/gpio-tangier.h
 
 INTEL GVT-g DRIVERS (Intel GPU Virtualization)
 M:	Zhenyu Wang <zhenyuw@linux.intel.com>