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>

MAINTAINERS

@@ -7031,6 +7031,7 @@ L:	linux-acpi@vger.kernel.org
 S:	Maintained
 F:	Documentation/firmware-guide/acpi/gpio-properties.rst
 F:	drivers/gpio/gpiolib-acpi.c
+F:	drivers/gpio/gpiolib-acpi.h
 
 GPIO IR Transmitter
 M:	Sean Young <sean@mess.org>