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/encoder/EncoderInterface.hxx

@@ -84,7 +84,7 @@ public:
 	 *
 	 * @param tag the tag object
 	 */
-	virtual void SendTag(gcc_unused const Tag &tag) {
+	virtual void SendTag([[maybe_unused]] const Tag &tag) {
 	}
 
 	/**