chore(interface): avoid gnu extensions in macros

In Standard C++, the `__VA_ARGS__` (`...`) part of variadic macros must
have at least one argument. `(void)` is a valid way to declare
parameterless functions, so we're using that here, even though it's not
idiomatic (but then again, nor is the whole macro).

src/audio/backend/alsink.h

@@ -49,8 +49,8 @@ public:
     uint getSourceId() const;
     void kill();
 
-    SIGNAL_IMPL(AlSink, finishedPlaying)
+    SIGNAL_IMPL(AlSink, finishedPlaying, void)
-    SIGNAL_IMPL(AlSink, invalidated)
+    SIGNAL_IMPL(AlSink, invalidated, void)
 
 private:
     OpenAL& audio;

src/audio/iaudiosink.h

@@ -107,8 +107,8 @@ public:
     virtual operator bool() const = 0;
 
 signals:
-    DECLARE_SIGNAL(finishedPlaying);
+    DECLARE_SIGNAL(finishedPlaying, void);
-    DECLARE_SIGNAL(invalidated);
+    DECLARE_SIGNAL(invalidated, void);
 };
 
 #endif // IAUDIOSINK_H