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 <[email protected]>

Author: vstinner

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