For declarations of Mix_QuitFluidSynth, Mix_InitFluidSynth.

This code was removed upstream in:

commit a65a84adb8a898a1479e026c3e3520b8ce95b728
Author: Sam Lantinga <slouken@libsdl.org>
Date:   Tue Oct 17 02:33:47 2017 -0700

    Rewrote music.c to support any number of decode libraries using a compiled-in plugin interface
    Mix_LoadWAV_RW() can now load sound formats that were previously available only as music.
    
    This is still work in progress. Testing and project updates need to happen on other platforms.


diff --git a/mixer.c b/mixer.c
index 2f28384474b9d4d1..f87d29a8bd3320bf 100644
--- a/mixer.c
+++ b/mixer.c
@@ -35,6 +35,7 @@
 #include "load_ogg.h"
 #include "load_flac.h"
 #include "dynamic_flac.h"
+#include "dynamic_fluidsynth.h"
 #include "dynamic_mod.h"
 #include "dynamic_mp3.h"
 #include "dynamic_ogg.h"