x11/alacritty: update libc crate to 0.2.39

alacritty >= g20180312 works fine as is on FreeBSD >= 12.0 as libc >= 0.2.38
uses pre-ino64 syscalls (via COMPAT_FREEBSD11). To simplify maintenance, update
libc and drop the patch, similar to what lang/rust-nightly did.

https://github.com/rust-lang/libc/commit/969ad2b73cdc
https://github.com/rust-lang/libc/commit/78f93220d70e

PR: 222824