acer-wmi: add Aspire 5741G touchpad toggle key

Add Aspire 5741G KEY_TOUCHPAD_TOGGLE to wmi keymap, preventing
"acer_wmi: Unknown key number - 0x85" error.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>