glib/win32: introduce private g_win32_handle_is_socket()

Used in following commits, including in some GIO experiments.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>

glib/glib-private.c

@@ -56,6 +56,7 @@ glib__private__ (void)
     g_win32_fstat,
     g_win32_find_helper_executable_path,
     g_win32_reopen_noninherited,
+    g_win32_handle_is_socket,
 #endif
   };