Fix mouse buttons (#441)

The following change in udev seems to incorrectly identify the virtual
pointer as a 'tablet pad'.

https://github.com/systemd/systemd/commit/0855ce6772#diff-e3c18b2fc4f50acbb9f2d66a1daf229fde99a5c8785f17aa6f69e6a9c41ae2c7R261

This patch removes the BTN_[0-9] capability from the virtual pointer (which is
presently unused) to avoid the issue. It is unclear if this behaviour should
be considered pathological in udev, since I can't find any documentation which
suggests it is illegitimate for non tablet devices to make use of these codes.

src/vkbd/uinput.c

@@ -124,9 +124,6 @@ static int create_virtual_pointer(const char *name)
 	for (code = BTN_LEFT; code <= BTN_TASK; code++)
 		ioctl(fd, UI_SET_KEYBIT, code);
 
-	for (code = BTN_0; code <= BTN_9; code++)
-		ioctl(fd, UI_SET_KEYBIT, code);
-
 	udev.id.bustype = BUS_USB;
 	udev.id.vendor = 0x0FAC;
 	udev.id.product = 0x1ADE;