configure: Fix compiler warning in config.log (unused variable)

warning: unused variable ‘iov’
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>

configure

@@ -2060,7 +2060,7 @@ cat > $TMPC <<EOF
 #include <sys/types.h>
 #include <sys/uio.h>
 #include <unistd.h>
-int main(void) { struct iovec iov; return 0; }
+int main(void) { return sizeof(struct iovec); }
 EOF
 iovec=no
 if compile_prog "" "" ; then