Input: synaptics - fix compile warning

Move synaptics_invert_y() inside CONFIG_MOUSE_PS2_SYNAPTICS to get rid of
a compile warning when we don't select synaptics support.

drivers/input/mouse/synaptics.c:53:12: warning: ‘synaptics_invert_y’ defined but not used [-Wunused-function]

Signed-off-by: JJ Ding <dgdunix@gmail.com>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>