gwin32.c: Split out call to RtlGetVersion()

Unfortunately, we may well be likely to need to call RtlGetVersion() via
GetModuleHandle() + GetProcAddress(), so split out the call to RtlGetVersion()
into a private function of its own, so that we can reuse the same code in other
parts of GLib, so that we can:

*  Determine better in a more fine-tuned way to determine whether we are on
   Windows 10/11 and/or Server 2016/2019/2022, since we need to rely on the
   build number.

*  Just call RtlGetVersion() once, when needed, as that is all that is needed.
   We could re-use the same function once to compare what we got when we
   called RtlGetVersion() and do what is necessary there.

glib/glib-init.h

@@ -39,6 +39,7 @@ void g_console_win32_init (void);
 void g_clock_win32_init (void);
 void g_crash_handler_win32_init (void);
 void g_crash_handler_win32_deinit (void);
+gboolean _g_win32_call_rtl_version (OSVERSIONINFOEXW *info);
 extern HMODULE glib_dll;
 #endif