rfkill-gpio: include linux/mod_devicetable.h

One more driver is apparently broken by the recent change
to linux/platform_device.h:

net/rfkill/rfkill-gpio.c: In function 'rfkill_gpio_acpi_probe':
net/rfkill/rfkill-gpio.c:82:29: error: dereferencing pointer to incomplete type 'const struct acpi_device_id'

Include linux/mod_devicetable.h to get the definition of the
acpi_device_id structure.

Fixes: ac316725 ("headers: separate linux/mod_devicetable.h from linux/platform_device.h")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>