configure: Fix errors in test for__sync_fetch_and_and

The old test code raises two compiler warnings which are errors since
commit 417c9d72.

These errors could result in compilations with compiler flag
-march486 (so all nice features of newer processors got lost).
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

@@ -2799,7 +2799,7 @@ fi
 # specification is necessary
 if test "$vhost_net" = "yes" && test "$cpu" = "i386"; then
   cat > $TMPC << EOF
-int sfaa(unsigned *ptr)
+static int sfaa(int *ptr)
 {
   return __sync_fetch_and_and(ptr, 0);
 }