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/playlist/plugins/EmbeddedCuePlaylistPlugin.cxx

@@ -82,7 +82,7 @@ ExtractCuesheetTagHandler::OnPair(StringView name, StringView value) noexcept
 
 static std::unique_ptr<SongEnumerator>
 embcue_playlist_open_uri(const char *uri,
-			 gcc_unused Mutex &mutex)
+			 [[maybe_unused]] Mutex &mutex)
 {
 	if (!PathTraitsUTF8::IsAbsolute(uri))
 		/* only local files supported */

src/playlist/plugins/XspfPlaylistPlugin.cxx

@@ -79,7 +79,7 @@ static constexpr struct tag_table xspf_tag_elements[] = {
 
 static void XMLCALL
 xspf_start_element(void *user_data, const XML_Char *element_name,
-		   gcc_unused const XML_Char **atts)
+		   [[maybe_unused]] const XML_Char **atts)
 {
 	auto *parser = (XspfParser *)user_data;
 	parser->value.clear();