*: change C-style prototypes, drop "(void)"

src/output/OutputPlugin.hxx

@@ -44,7 +44,7 @@ struct AudioOutputPlugin {
 	 * Test if this plugin can provide a default output, in case
 	 * none has been configured.  This method is optional.
 	 */
-	bool (*test_default_device)(void);
+	bool (*test_default_device)();
 
 	/**
 	 * Configure and initialize the device, but do not open it

src/output/OutputState.hxx

@@ -41,6 +41,6 @@ audio_output_state_save(BufferedOutputStream &os,
  * whether the state has changed and the state file should be saved.
  */
 unsigned
-audio_output_state_get_version(void);
+audio_output_state_get_version();
 
 #endif