MAINTAINERS: Append missed file to the database

When gpiolib.h internal header had been split to few, the commit 77cb907a
("gpiolib: acpi: Split ACPI stuff to gpiolib-acpi.h") in particular missed
the MAINTAINERS database update. Do it here.

Fixes: 77cb907a

 ("gpiolib: acpi: Split ACPI stuff to gpiolib-acpi.h")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>