audio/sdlaudio: Allow audio playback with SDL2

When compiling with SDL2, the semaphore trick used in sdlaudio.c
does not work - QEMU locks up completely in this case. To avoid
the hang and get at least some audio playback up and running (it's
a little bit crackling, but better than nothing), we can use the
SDL locking functions SDL_LockAudio() and SDL_UnlockAudio() to sync
with the sound playback thread instead.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-id: 1485852398-2327-1-git-send-email-thuth@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>