x11-fonts/encodings: run mkfontscale to fix encodings.dir post-install

• Since only *.enc.gz files are installed while the encodings.dir file is genareted for directories containing both *.enc and *.enc.gz files, the installed encodings.dir file includes bogus entries for *.enc.
• It causes libfontenc to fail to find encoding files, then X to fail to open TrueType fonts with some encodings such as ascii-0 or jisx0208.1990-0.

PR: 170852
Submitted by: hiroto.kagotani@gmail.com, Jia-Shiun Li <jiashiun@gmail.com>