Fix Address of stack memory associated with local variable 'flag' is still

referred to by the global variable 'long_options' upon returning to the caller.
This will be a dangling reference

Found by Clang Analyzer

src/h2load.cc

@@ -638,7 +638,7 @@ Options:
 int main(int argc, char **argv)
 {
   while(1) {
-    int flag = 0;
+    static int flag = 0;
     static option long_options[] = {
       {"requests", required_argument, nullptr, 'n'},
       {"clients", required_argument, nullptr, 'c'},