[clang-tidy] Pass by value with std::move

Found with modernize-pass-by-value

Signed-off-by: Rosen Penev <rosenp@gmail.com>

src/Variant.h

@@ -47,7 +47,7 @@ public:
   Variant (const bool);
   Variant (const int);
   Variant (const double);
-  Variant (const std::string&);
+  Variant (std::string );
   Variant (const char*);
   Variant (const time_t, const enum type);