monkeysAudio: Fix compilation using an older version of gcc

Fixes this issue:

> /nix/store/...-gcc-11.2.0/include/c++/11.2.0/cmath:1930:1: error: expected declaration before '}' token
>  1930 | } // extern "C++"
>       | ^

pkgs/applications/audio/monkeys-audio/default.nix

@@ -1,6 +1,6 @@
-{lib, stdenv, fetchurl}:
+{lib, gcc10Stdenv, fetchurl}:
 
-stdenv.mkDerivation rec {
+gcc10Stdenv.mkDerivation rec {
   version = "3.99-u4-b5";
   pname = "monkeys-audio";