remove gcc_unused

[[maybe_unused]] (introduced in C++17) is standard C++.

https://clang.llvm.org/docs/AttributeReference.html#maybe-unused-unused
says that this is equivalent to the GNU unused attribute.

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

src/output/plugins/sles/SlesOutputPlugin.cxx

@@ -111,7 +111,7 @@ private:
 	 * been consumed.  It synthesises and enqueues the next
 	 * buffer.
 	 */
-	static void PlayedCallback(gcc_unused SLAndroidSimpleBufferQueueItf caller,
+	static void PlayedCallback([[maybe_unused]] SLAndroidSimpleBufferQueueItf caller,
 				   void *pContext)
 	{
 		SlesOutput &sles = *(SlesOutput *)pContext;