qemu-kvm: fix pulseaudio detection in configure

pulse/simple.h does not include stdlib.h
We cannot use NULL since it may not be defined
Use 0 instead
Signed-off-by: NMarc-Antoine Perennou <Marc-Antoine@Perennou.com>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>

configure

@@ -1638,7 +1638,7 @@ for drv in $audio_drv_list; do
 
     pa)
     audio_drv_probe $drv pulse/simple.h "-lpulse-simple -lpulse" \
-        "pa_simple *s = NULL; pa_simple_free(s); return 0;"
+        "pa_simple *s = 0; pa_simple_free(s); return 0;"
     libs_softmmu="-lpulse -lpulse-simple $libs_softmmu"
     audio_pt_int="yes"
     ;;