package/sdl2_mixer: fix fluidsynth option

Option to disable fluidsynth is --disable-music-midi-fluidsynth since
version 2.0.0 and
https://github.com/SDL-mirror/SDL_mixer/commit/9fe78ec49f083e4f85e10abb9f1cbe7d6187d6b0



Before this commit, the configure option was
--disable-music-fluidsynth-midi (and not --disable-fluidsynth)

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>