ALL included files must be the same type.
Either *.C or *.WDL
So assuming you stick to *.WDL's with action.WDL as your primary project file,
that is, its the one you run and it contains the
function main() function.
File: actions.wdl
action pl_rudi();
File: player.wdl
action pl_rudi()
{
//do something here
}
OR
//because ACTION and VOID functions are identical to the engine
File: actions.c
void pl_rudi();
File: player.c
void pl_rudi()
{
//do something here
}