msgpack: rename from libmsgpack

The project does not use the name "libmsgpack". It variously uses "msgpack" and
"msgpack-c", and "msgpackc" for the name of the library.

pkgs/development/libraries/libtoxcore/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchFromGitHub, cmake, libsodium, ncurses, libopus, libmsgpack
+{ stdenv, fetchFromGitHub, cmake, libsodium, ncurses, libopus, msgpack
 , libvpx, check, libconfig, pkgconfig }:
 
 let
@@ -20,7 +20,7 @@ let
     ];
 
     buildInputs = [
-      libsodium libmsgpack ncurses libconfig
+      libsodium msgpack ncurses libconfig
     ] ++ stdenv.lib.optionals (!stdenv.isAarch32) [
       libopus libvpx
     ];