Make ff-find-other-file symmetric for C++ (Bug#20192)

Make ff-find-other-file symmetric for C++ (Bug#20192)

`cc-other-file-alist' has a mapping of file extensions to switch
between headers and sources, but the mappings weren't completely
symmetric.  In particular .cpp would map to .hh, but .hh would NOT map
to .cpp.

* lisp/find-file.el (cc-other-file-alist): Map ".hh" and ".h" to all
C++ extensions to make them symmetric with the C++ extensions that map
to them.  This lets repeated invocations of `ff-find-other-file'
toggle between all pairs of sources/headers.