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/input/plugins/AlsaInputPlugin.cxx

@@ -110,7 +110,7 @@ protected:
 		InvalidateSockets();
 	}
 
-	void DoSeek(gcc_unused offset_type new_offset) override {
+	void DoSeek([[maybe_unused]] offset_type new_offset) override {
 		/* unreachable because seekable==false */
 		SeekDone();
 	}