windows: determine RtlVerifyVersionInfo address on global init

Instead of the first internal call to `curlx_verify_windows_version()`.

To avoid the chance of a race, potentially resulting in initializing
this address twice. AFAICT it could not cause an issue before this
patch.

Reported by Codex Security

Follow-up to b17ef873ae #18009

Closes #20853

src/tool_doswin.c

@@ -901,6 +901,7 @@ curl_socket_t win32_stdin_read_thread(void)
 
 CURLcode win32_init(void)
 {
+  curlx_verify_windows_init();
   curlx_now_init();
 #ifndef CURL_WINDOWS_UWP
   init_terminal();