AUDIO: Fix ReportHandlerScummVM::printDebug in MT32 code.

Formerly it tried to pass 'va_list' as '...' to debug. Now it's first creating
a String containing the output via String::vformat and then passing it
to debug.

Found by buildbot (master-dc). The warning message was:
audio/softsynth/mt32.cpp:65: warning: cannot pass objects of non-POD type
`struct va_list' through `...'; call will abort at runtime