thinkpad_acpi: Fix warning for static not at beginning

Fix the following warning:

warning: "static" is not at beginning of declaration
 void static hotkey_mask_warn_incomplete_mask(void)
 ^

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Henrique de Moraes Holschuh <ibm-acpi@hmh.eng.br>
Cc: Darren Hart <dvhart@infradead.org>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>