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: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

configure

diff --git a/configure b/configure
index 4fa9a57..a311071 100755
--- a/configure
+++ b/configure
@@ -1638,7 +1638,7 @@
 
     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"
     ;;