optarg is from unistd.h, use opt_arg from apr_getopt().

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1916148 13f79535-47bb-0310-9956-ffa450edef68

support/ab.c

@@ -3068,7 +3068,7 @@ int main(int argc, const char * const argv[])
                 keepalive = 1;
                 break;
             case 'o':
-                src_address = strdup(optarg);
+                src_address = strdup(opt_arg);
                 break;
             case 'q':
                 heartbeatres = 0;