Make ecpg's rjulmdy() and rmdyjul() agree with their declarations.

Make ecpg's rjulmdy() and rmdyjul() agree with their declarations.

We had "short *mdy" in the extern declarations, but "short mdy[3]"
in the actual function definitions.  Per C99 these are equivalent,
but recent versions of gcc have started to issue warnings about
the inconsistency.  Clean it up before the warnings get any more
widespread.

This back-patches commit 1b242f42b into out-of-support branches,
pursuant to newly-established project policy.  The point is to
suppress scary-looking warnings so that people building these
branches needn't expend brain cells verifying that it's safe
to ignore the warnings.

Discussion: https://postgr.es/m/d0316012-ece7-7b7e-2d36-9c38cb77cb3b@enterprisedb.com