Fix -Wparentheses in track_cleaner.cpp

Changes "wxBusyCursor( dummy );" to just "wxBusyCursor dummy;"

Fixes warning: ../pcbnew/tracks_cleaner.cpp:155:17:
warning: unnecessary parentheses in declaration of ‘dummy’ [-Wparentheses]
     wxBusyCursor( dummy )

pcbnew/tracks_cleaner.cpp

@@ -152,7 +152,7 @@ void PCB_EDIT_FRAME::Clean_Pcb()
     // Old model has to be refreshed, GAL normally does not keep updating it
     Compile_Ratsnest( NULL, false );
 
-    wxBusyCursor( dummy );
+    wxBusyCursor dummy;
     BOARD_COMMIT commit( this );
     TRACKS_CLEANER cleaner( GetBoard(), commit );