makepkg.conf: Increase _FORTIFY_SOURCE level to 3
This is the relevant change for RFC 0017-increase-fortification-level that was merged 4 months ago.
Fedora and OpenSUSE also switched to level 3 in 2023.
Relevant TODO list: https://archlinux.org/todo/prepare-packages-for-d_fortify_source3/
Edited by kpcyrd