WIIU: Fix touchscreen mouse emulation

== DETAILS
The way the mouse emulation worked was to simply return a 3rd axis from the
gamepad, which the polling code updates with the touch state in real time.

Well, the code that figures out if it's a positive or negative axis was
failing because it didn't see that 3rd axis as valid. So, I added values
which allow it to be seen as valid, and voila! the touch screen input
works again.

== TESTING
Tested locally using "Beneath a Steel Sky" in ScummVM.

input/common/input_hid_common.c

@@ -22,6 +22,8 @@ enum gamepad_pad_axes
    AXIS_LEFT_ANALOG_Y,
    AXIS_RIGHT_ANALOG_X,
    AXIS_RIGHT_ANALOG_Y,
+   AXIS_TOUCH_X,
+   AXIS_TOUCH_Y,
    AXIS_INVALID
 };