net/zebra-pj missing autoheader

[Munechika Sumikawa]

> The port net/zebra-pj needs 'autoheader' installed in order to build.
> 
> This is part of devel/autoconf253, but this is missing from the
> BUILD_DEPENDS line of the Makefile.
> 
> Also, the full path to 'automake' isn't necessary.

Thanks, I've committed your patch.

-- Sumikawa