Fix a mixed declaration warning in the grpc_rb_call_get_peer_cert method

src/ruby/ext/grpc/rb_call.c

@@ -227,6 +227,7 @@ static VALUE grpc_rb_call_get_peer_cert(VALUE self) {
     return Qnil;
   }
 
+  {
     grpc_auth_property_iterator it =
         grpc_auth_context_find_properties_by_name(ctx, GRPC_X509_PEM_CERT_PROPERTY_NAME);
     const grpc_auth_property *prop = grpc_auth_property_iterator_next(&it);
@@ -235,6 +236,7 @@ static VALUE grpc_rb_call_get_peer_cert(VALUE self) {
     }
 
     res = rb_str_new2(prop->value);
+  }
 
   grpc_auth_context_release(ctx);