msgpack-cxx: add `msgpack-cpp` alias
msgpack-cxx: add `msgpack-cpp` alias

Most other related formulae in Homebrew/core are suffixed with `-cpp`
rather than `-cxx`. Also, the upstream branch this formula is built on
is called `cpp_master`, despite the actual artefact being called
`msgpack-cxx`.

This makes it likely that some users are going to try to do `brew
install msgpack-cpp` and end up confused by the resulting error. Let's
try to avoid that by adding this alias.

Moisan/homebrew-core,mfikes/homebrew-core,adam-moss/homebrew-core,stgraber/homebrew-core,nbari/homebrew-core,makigumo/homebrew-core,nandub/homebrew-core,zyedidia/homebrew-core,cblecker/homebrew-core,filcab/homebrew-core,sjackman/homebrew-core,Homebrew/homebrew-core,nbari/homebrew-core,Moisan/homebrew-core,mfikes/homebrew-core,sjackman/homebrew-core,chrisfinazzo/homebrew-core,lemzwerg/homebrew-core,filcab/homebrew-core,chrisfinazzo/homebrew-core,stgraber/homebrew-core,uyjulian/homebrew-core,adam-moss/homebrew-core,uyjulian/homebrew-core,makigumo/homebrew-core,cblecker/homebrew-core,nandub/homebrew-core,Homebrew/homebrew-core,zyedidia/homebrew-core,lemzwerg/homebrew-core