- cleaned i_music.h off external dependencies.

* take I_BuildMIDIMenuList out of it
* pass the command line state to disable music from the init call.
* don't declare internal 'nomusic' variable in the header.

src/common/menu/menudef.cpp

@@ -1662,6 +1662,7 @@ static void InitMusicMenus()
 // Special menus will be created once all engine data is loaded
 //
 //=============================================================================
+void I_BuildMIDIMenuList(FOptionValues*);
 
 void M_CreateMenus()
 {