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

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>

Misc/NEWS.d/next/Build/2023-09-02-18-04-15.gh-issue-63760.r8hJ6q.rst

@@ -0,0 +1,3 @@
+Fix Solaris build: no longer redefine the ``gethostname()`` function. Solaris
+defines the function since 2005. Patch by Victor Stinner, original patch by
+Jakub Kulík.

Modules/socketmodule.c

@@ -111,11 +111,6 @@ Local naming conventions:
 #include "pycore_fileutils.h"     // _Py_set_inheritable()
 #include "pycore_moduleobject.h"  // _PyModule_GetState
 
-// gethostname() prototype missing from Solaris standard header files
-#ifdef __sun
-extern int gethostname(char *, int);
-#endif
-
 #ifdef _Py_MEMORY_SANITIZER
 #  include <sanitizer/msan_interface.h>
 #endif