AppVeyor: Add a workaround for build with Visual Studio 2019 on ARM64

Avoid the SDK 10.0.26100.0. Use an older SDK: 10.0.22621.0.

Use when needed:
PLATFORM: ARM64,version=10.0.22621.0

The problem first appeared on 2025-11-07, after the 2025-11-06
image update (Visual Studio 2019/2022).

The errors were:
C:\Program Files (x86)\Windows Kits\10\Include\10.0.26100.0\um\winnt.h
(6343,27): warning C4013: '_CountOneBits64' undefined; assuming extern
returning int

When it was OK:
cmake version 4.1.0
-- The C compiler identification is MSVC 19.29.30159.0

Current:
cmake version 4.1.0
-- Selecting Windows SDK version 10.0.26100.0 to target Windows 10.0.17763.
-- The C compiler identification is MSVC 19.29.30159.0

[skip ci]

Author: fxlb

.appveyor.yml

@@ -92,13 +92,13 @@ environment:
       # VS 2019, Npcap, 64-bit ARM, without remote
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
       GENERATOR: "Visual Studio 16 2019"
-      PLATFORM: ARM64
+      PLATFORM: ARM64,version=10.0.22621.0
       SDK: npcap-sdk
       REMOTE: -DENABLE_REMOTE=NO
       # VS 2019, Npcap, 64-bit ARM, with remote
     - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
       GENERATOR: "Visual Studio 16 2019"
-      PLATFORM: ARM64
+      PLATFORM: ARM64,version=10.0.22621.0
       SDK: npcap-sdk
       OPENSSL_ROOT_DIR: -DOPENSSL_ROOT_DIR=C:\OpenSSL-v33-Win64\bin
       # VS 2022, WinPcap, 32-bit and 64-bit x86, without remote