x11/libX11: Update to 1.6.12

Update x11/libX11 to 1.6.12, which contains a fix for a double free
vulnerability.
The update also contains fixes already included in the FreeBSD libX11 port.

Announcement and changelog:
https://lists.x.org/archives/xorg-announce/2020-August/003057.html

MFH: 2020Q3 (implicit, security fix)
Security: 8da79498-e6f6-11ea-8cbf-54e1ad3d6335