[ox/std] Add missing declaration for String::String(const String&)

(synced from aa1b5fa4cf)

src/ox/std/string.hpp

@@ -27,6 +27,8 @@ class String {
 
 		String(const char *str) noexcept;
 
+		String(String&) noexcept;
+
 		const String &operator=(const char *str) noexcept;
 
 		const String &operator=(char *str) noexcept;