fix implicitly declared operator= problem

src/core/proteinxtp.cpp

@@ -39,6 +39,15 @@ DbXref::DbXref(const DbXref &other)
   : database(other.database), accession(other.accession)
 {
 }
+
+DbXref &
+DbXref::operator=(const DbXref &other)
+{
+  database  = other.database;
+  accession = other.accession;
+  return *this;
+}
+
 const QUrl
 DbXref::getUrl() const
 {

src/core/proteinxtp.h

@@ -38,6 +38,7 @@ struct DbXref
   DbXref();
   DbXref(const DbXref &other);
   DbXref(ExternalDatabase database_in, const QString &accession_in);
+  DbXref &operator=(const DbXref &);
   const QUrl getUrl() const;
   ExternalDatabase database;
   QString accession;