configure: Link QEMU against 'liburcu-bp'

This library is needed when using 'ust/tracepoint.h'.
Signed-off-by: NHarsh Prateek Bora <harsh@linux.vnet.ibm.com>
Signed-off-by: NLluís Vilanova <vilanova@ac.upc.edu>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>

configure

@@ -2672,7 +2672,7 @@ if test "$trace_backend" = "ust"; then
 int main(void) { return 0; }
 EOF
   if compile_prog "" "" ; then
-    LIBS="-lust $LIBS"
+    LIBS="-lust -lurcu-bp $LIBS"
   else
     echo
     echo "Error: Trace backend 'ust' missing libust header files"