[mlir] Fix gcc-5 build in ViewOpGraph.cpp

[ftynse]

Feel free to push build breakage fixes directly.

mlir/lib/Transforms/ViewOpGraph.cpp

private:
   std::string attrStmt(const Twine &key, const Twine &value) {
     return (key + " = " + value).str();
   }
 
   /// Emit an attribute list.
   void emitAttrList(raw_ostream &os, const AttributeMap &map) {
     os << "[";
     interleaveComma(map, os, [&](const auto &it) {
-      os << attrStmt(it.getKey(), it.getValue());
+      os << this->attrStmt(it.getKey(), it.getValue());
     });
     os << "]";
   }
 
   // Print an MLIR attribute to `os`. Large attributes are truncated.
   void emitMlirAttr(raw_ostream &os, Attribute attr) {
     // A value used to elide large container attribute.
     int64_t largeAttrLimit = getLargeAttributeSizeLimit();