fix for bug #47381, how to order #includes

[Mike Miller]

Rik, all,

The fix [1] for bug #47381 [2] accidentally breaks building Octave
without gl2ps [3]. The header file associated with gl2ps-print.cc,
gl2ps-print.h, needs to be #included outside of the HAVE_GL2PS_H
condition.

This is easy to restore, but any ideas how to avoid accidentally
rearranging things like this in the future? I'd be tempted to clean up
includes in the same way and make the same mistake. The graphics classes
are particularly complicated. Should we comment when #includes are
grouped for particular reasons like this? Should we use additional #if
blocks to separate conditional includes from unconditional includes, and
conditional code from unconditional code?

[1]: http://hg.savannah.gnu.org/hgweb/octave/rev/ab8760b1245d
[2]: https://savannah.gnu.org/bugs/?47381
[3]: https://hydra.nixos.org/build/33552213

-- 
mike