Several functions in server/ were not specifying an argument list,
ie they were declared as void foo(); When compiling with
-Wstrict-prototypes, this leads to:
test_playback.c:93:5: erreur: function declaration isn’t a prototype
[-Werror=strict-prototypes]
and spice_server_playback_put_samples. The former retrieves a buffer from a free
list with spice_server_playback_get_buffer, and should be used once via
spice_server_playback_put_samples. The tester previously reused the same buffer
a number of times.