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/db/Interface.hxx

@@ -126,8 +126,8 @@ public:
 	 *
 	 * @return the job id or 0 if not implemented
 	 */
-	virtual unsigned Update(gcc_unused const char *uri_utf8,
-				gcc_unused bool discard) {
+	virtual unsigned Update([[maybe_unused]] const char *uri_utf8,
+				[[maybe_unused]] bool discard) {
 		/* not implemented: return 0 */
 		return 0;
 	}