[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/filter/plugins/VolumeFilterPlugin.cxx

@@ -34,7 +34,7 @@ public:
 						  true);
 	}
 
-	unsigned GetVolume() const noexcept {
+	[[nodiscard]] unsigned GetVolume() const noexcept {
 		return pv.GetVolume();
 	}