gh-63760: Don't declare gethostname() on Solaris (#108817)

Since 2005, Solaris defines gethostname(). socketmodule.c no longer
has to define gethostname() for Solaris.

Oracle Solaris and OpenSolaris have patches to remove the
gethostname() definition in Python:

* https://github.com/oracle/solaris-userland/blob/master/components/python/python37/patches/15-gethostname.patch
* https://github.com/OpenIndiana/oi-userland/blob/oi/hipster/components/python/python37/patches/15-gethostname.patch
* https://github.com/omniosorg/omnios-build/blob/master/build/python27/patches/24-gethostname.patch

Co-authored-by: Jakub Kulík <Kulikjak@gmail.com>