[flang][runtime] Fix error message

[klausler]

Summary

A literal % character in an error message needs to be doubled
so that it appears in the output of snprintf().

flang/runtime/namelist.cpp

if (const typeInfo::DerivedType *
           name);
     } else {
       handler.SignalError("NAMELIST component reference '%%%s' of input group "
                           "item %s for non-derived type",
           compName, name);
     }
   } else {
     handler.SignalError("NAMELIST component reference of input group item %s "
-                        "has no name after '%'",
+                        "has no name after '%%'",
         name);
   }
   return false;
 }
 
 // Advance to the terminal '/' of a namelist group.
 static void SkipNamelistGroup(IoStatementState &io) {
   std::size_t byteCount{0};