Ports: Create launchers for the stpuzzles port

This changes the .port_include.sh script so that ports can more easily
create more than one launcher by making the install_launcher function
available to the port's package.sh script.

This creates launchers for the stpuzzles port in the Games/Puzzles
category.

Ports/openttd/package.sh

@@ -47,6 +47,4 @@ install() {
     )
 
     ln -sf /usr/local/games/openttd $DESTDIR/usr/local/bin/openttd
-
-    install_launcher
 }