Hi

`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.  This patch updates the mappings to always contain symmetric
pairs so that repeated invocations of (ff-find-other-file) can toggle
between all pairs of sources/headers.