message: Fix windows build broken by r334004
  The problem was a link error due to a missing =0 on an abstract method.
  Interestingly, this was not a problem for clang/linux.
diff: @@ -39,7 +39,7 @@ public:
  virtual void GetFunctions(ConstString name, DWARFDebugInfo &info,
  const CompilerDeclContext &parent_decl_ctx,
  uint32_t name_type_mask,
  - std::vector<DWARFDIE> &dies);
  + std::vector<DWARFDIE> &dies) = 0;
  virtual void GetFunctions(
  const RegularExpression &regex, DWARFDebugInfo &info,
  llvm::function_ref<bool(const DWARFDIE &die, bool include_inlines,
  