[clang-tidy] use nodiscard

Introduced in C++17. It replaces gcc's warn_unused_result.

Found with modernize-use-nodiscard.

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

src/db/plugins/ProxyDatabasePlugin.cxx

@@ -61,7 +61,7 @@ public:
 	LibmpdclientError(enum mpd_error _code, const char *_msg)
 		:std::runtime_error(_msg), code(_code) {}
 
-	enum mpd_error GetCode() const {
+	[[nodiscard]] enum mpd_error GetCode() const {
 		return code;
 	}
 };