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

Since at least 2013, Oracle 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. Oracle
+Solaris defines the function since at least 2013. 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