player: fix -Wunused-parameter warning

Fixes

  ../engine/player/sc_player.cpp:2990:47: warning: unused parameter 'a' [-Wunused-parameter]
  void player_t::action_init_finished(action_t& a)
                                                ^

with clang HEAD.

engine/player/sc_player.cpp

@@ -2987,7 +2987,7 @@ void player_t::init_finished()
   }
 }
 
-void player_t::action_init_finished(action_t& a)
+void player_t::action_init_finished(action_t&)
 {
   // here could be generic affected_by code
 }