gpiomon: initialize the active low variable

gpiomon: initialize the active low variable

The variable active_low is not initialized, so the value is undefined
and if the -l argument is not passed, the undefined value remains. Fix
it by initializing it to false.

Signed-off-by: Clemens Gruber <clemens.gruber@pqgruber.com>
Signed-off-by: Bartosz Golaszewski <bartekgola@gmail.com>