Add DIERef::operator==

FIXME - verify that this was needed, and by what.

Author: tromey

source/Plugins/SymbolFile/DWARF/DIERef.h

@@ -45,6 +45,8 @@ struct DIERef {
 
   bool operator<(const DIERef &ref) { return die_offset < ref.die_offset; }
 
+  bool operator==(const DIERef &ref) const { return die_offset == ref.die_offset; }
+
   dw_offset_t cu_offset;
   dw_offset_t die_offset;
 };