Mk/Uses/kde.mk: set up UTF-8 locale as default on build environment (+)

Recently we discovered a bug with plasma apps parsing non-latin strings when
pure C locale was used at compile stage which leads to save and display
broken strings full of "?" symbols.

Fix it by set a full UTF-8 locale for the build process.

Idea: arrowd
Tests: fluffy

Sponsored by: Serenity Cybersecurity, LLC