test: update midifile library to ANSI C

test: update midifile library to ANSI C

The midifile library used by the playmidi1 program dates from 1989, and
used pre-ANSI function definitions and prototypes. GCC 15 now defaults
to C23 where () means the same as (void) in prototypes, which causes
type mismatch errors.

Update the code to use ANSI function definitions and prototypes, so
it'll compile happily as anything from ANSI C to C23. This revealed that
playmidi1's do_tempo had the wrong argument type, so correct that as
well.

Closes: https://github.com/alsa-project/alsa-lib/pull/463
Signed-off-by: Adam Sampson <ats@offog.org>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>