mumble: Fix build with boost version 1.66

This is already tracked in upstream issue mumble-voip/mumble#3281 and a
fix has been merged in mumble-voip/mumble@caa187373ec2f8bcf5b88e6340973.

The patch I'm adding here is using the merged commit
mumble-voip/mumble@ea861fe86743c8402bbad77d8d1dd9de8dce447e and I've
only added it for the stable release because the patch is already
included in the git version.

@pbogdan also had a similar commit to this
(pbogdan/nixpkgs@8029edea298b36df6494885055c8bf123aa4d26b), but the
patch was applied to both stable and git and thus the git version would
have been broken.

Tested by building mumble and mumble_git and running the mumble NixOS VM
test.

Signed-off-by: aszlig <aszlig@nix.build>
Cc: @viric, @jgeerds, @abbradar
Fixes: #33655