configure: Fix build with ALSA audio driver

Since commit 417c9d72,
all configure tests normally run with -Werror.

Some of these tests now fail because they raised a compiler warning.

Here a build breakage for ALSA (configure --audio-drv-list=alsa) is fixed.
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

configure

@@ -1891,7 +1891,7 @@ for drv in $audio_drv_list; do
     case $drv in
     alsa)
     audio_drv_probe $drv alsa/asoundlib.h -lasound \
-        "snd_pcm_t **handle; return snd_pcm_close(*handle);"
+        "return snd_pcm_close((snd_pcm_t *)0);"
     libs_softmmu="-lasound $libs_softmmu"
     ;;