Update toolchain.h

Author: coder137

buildcc/lib/toolchain/include/toolchain/toolchain.h

@@ -58,6 +58,9 @@ class Toolchain : public ToolchainVerify<Toolchain> {
   const std::string &GetArchiver() const { return archiver_; }
   const std::string &GetLinker() const { return linker_; }
 
+private:
+  friend class ToolchainVerify<Toolchain>;
+
 private:
   Id id_;
   std::string name_;