Fix the following sparse warning:

thinkpad_acpi.c:3459:11: warning: symbol 'adaptive_keyboard_modes' was not declared. Should it be static?
Signed-off-by: NLad, Prabhakar <prabhakar.csengg@gmail.com>
Acked-by: NHenrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: NDarren Hart <dvhart@linux.intel.com>