package/webkitgtk: disable gamepad support

Pass -DENABLE_GAMEPAD=OFF to CMake in order to disable support for the
gamepad API, which requires libmanette, a library that is not yet
available in Buildroot.

Signed-off-by: Adrian Perez de Castro <aperez@igalia.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>