x11-drivers/xf86-input-synaptics: enable EVDEV by default on FreeBSD 12+

After switching default value of kern.evdev.rcpt_mask sysctl to 12 (r360126)
some users get their touchpads broken as they have xf86-input-synaptics port
installed. See e.g. PR/245948. This happens as xf86-input-synaptics has
greater priority than libinput and effectively disables latter after
installation. Workaround this issue with enabling EVDEV support in the port.

PR: 245966
Approved by: x11 (zeising)