Input: psmouse - fix build error of multiple definition

trackpoint_detect() should be static inline while
CONFIG_MOUSE_PS2_TRACKPOINT is not set, otherwise, we build fails:

drivers/input/mouse/alps.o: In function `trackpoint_detect':
alps.c:(.text+0x8e00): multiple definition of `trackpoint_detect'
drivers/input/mouse/psmouse-base.o:psmouse-base.c:(.text+0x1b50): first defined here

Reported-by: Hulk Robot <hulkci@huawei.com>
Fixes: 55e3d922 ("Input: psmouse - allow disabing certain protocol extensions")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>