variable.h missing from po/POTFILES.in

[Henning Makholm]

In make 3.80, the code to emit warnings about undefined variables
(with --warn-undefined-variables) has moved from expand.c (I think)
to variable.h - but variable.h has not been added to po/POTFILES.in.
This means that those warnings do not get properly internationalized.

-- 
Henning Makholm                   "The great secret, known to internists and
                         learned early in marriage by internists' wives, but
               still hidden from the general public, is that most things get
         better by themselves. Most things, in fact, are better by morning."