input-raw: Handle BTN_LEFT like BTN_TOUCH

This is needed for an eGalax HID touch screen that is configured to
emulate mouse input (vendor 0x0eef product 0x731a) and as such supports
BTN_LEFT but not BTN_TOUCH.

Signed-off-by: Forest Bond <forest.bond@rapidrollout.com>
Merge pull request #15 from forest-bond/master

input-raw: Handle BTN_LEFT like BTN_TOUCH