.cc files not including .h files

[Rik]

jwe,

I didn't really want to get in to the mess of streamlining the header
files, but I stumbled upon data.cc in libinterp/corefcn which does not
include data.h.  This seems odd as the pattern is that a file XXX.cc
include XXX.h if such a header file is available.  I wrote a short Perl
script to check this, and, for the libinterp directory, I see the following
cc files don't include the equivalently named header file.

data.cc
defaults.cc
ft-text-renderer.cc
mex.cc
oct-hdf5-types.cc
toplev.cc

Seems like something that should be reviewed and maybe a comment made if
this is correct.  For example, mex.h is a C-only include file.  However,
there is already a note there that says it should not be included in mex.cc.

--Rik