Fix locales not being loaded when RUN_IN_PLACE=0

When minetest is installed system-wide (on unix-like systems), locales aren't loaded, the game keeps being in english.
This is because locales are installed to share/locales, and the game tries to load them from share/minetest/locales.

This simple fix just installs them to share/minetest/locales, but you might think it would be better to fix it in source code so they are loaded from share/locales