x11-fonts/mkfontscale: Update to 1.2.1

Update x11-fonts/mkfontscale to 1.2.1.

This release fixes a regression introduced in the 1.2.0 release that could
leave the mkfontdir script containing an unexpanded reference to
${exec_prefix}.

Changelog:
https://lists.x.org/archives/xorg-announce/2019-March/002988.html