games/scummvm: Unbreak build with FLUIDSYNTH=on

After commit 4c2db26adf7d the build, if the non-default option
FLUIDSYNTH is enabled, fails with:

In file included from audio/softsynth/fluidsynth.cpp:32:
In file included from /usr/local/include/fluidsynth.h:100:
[..]
audio/softsynth/fluidsynth.cpp:252:3: error:

no matching function for call to 'fluid_sfloader_set_callbacks'
             fluid_sfloader_set_callbacks(soundFontMemoryLoader,
             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~

/usr/local/include/fluidsynth/sfont.h:172:20: note:

candidate function not viable:
no known conversion from 'int (void *, int, void *)' to
'fluid_sfloader_callback_read_t' (aka 'int (*)(void *, long long, void *)') for 3rd argument

FLUIDSYNTH_API int fluid_sfloader_set_callbacks(fluid_sfloader_t *loader,

^

Obtained from: NetBSD
MFH: 2021Q4

(cherry picked from commit 05a45a5e50775e34b7c949ee3c6607dbb004b3a1)