Revert the icon name in minetest.desktop

This reverts commit 7993696f, which broke the
icon on most distributions.

misc/minetest.desktop

@@ -3,7 +3,7 @@ Name=Minetest
 GenericName=Minetest
 Comment=Multiplayer infinite-world block sandbox
 Exec=minetest
-Icon=minetest
+Icon=minetest-icon
 Terminal=false
 Type=Application
 Categories=Game;