Definitions of `gMusicPlayer_MainBgm` & `gMusicPlayer_FightBgm` are reversed

[laqieer]

The 1st music player is for BGM, and the 2nd music player is for map music, so gMusicPlayer_MainBgm is gMusicPlayer_FightBgm, and gMusicPlayer_FightBgm is gMusicPlayer_MainBgm.