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/lib/upnp/Action.hxx

@@ -32,7 +32,7 @@ CountNameValuePairs() noexcept
 
 template<typename... Args>
 static constexpr unsigned
-CountNameValuePairs(gcc_unused const char *name, gcc_unused const char *value,
+CountNameValuePairs([[maybe_unused]] const char *name, [[maybe_unused]] const char *value,
 		    Args... args) noexcept
 {
 	return 1 + CountNameValuePairs(args...);