Fix build error with clang-cl on latest Windows SDK

Found when testing windows-2025 runner in GHA.
So switch to that to make sure the error is fixed.

Change-Id: I3046f4b09a4700001a1d9dcce802990dfa701b52
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <20250804195054.21729-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg32513
Signed-off-by: Gert Doering <gert@greenie.muc.de>

.github/workflows/build.yaml

@@ -116,7 +116,7 @@ jobs:
         arch: [x86, x64]
         test: [argv, auth_token, buffer, cryptoapi, crypto, misc, ncp, packet_id, pkt, provider, ssl, tls_crypt, user_pass]
 
-    runs-on: windows-latest
+    runs-on: windows-2025
     name: "mingw unittest ${{ matrix.test }} - ${{ matrix.arch }} - OSSL"
     steps:
       - name: Checkout OpenVPN
@@ -264,7 +264,7 @@ jobs:
       env:
         BUILD_CONFIGURATION: Release
 
-      runs-on: windows-latest
+      runs-on: windows-2025
       steps:
       - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
       - uses: lukka/get-cmake@6b3e96a9bc9976b8b546346fdd102effedae0ca8 # v4.0.3

src/openvpn/syshead.h

@@ -44,7 +44,7 @@
 #define srandom  srand
 #endif
 
-#ifdef _MSC_VER /* Visual Studio */
+#if defined(_MSC_VER) && !defined(__clang__) /* Microsoft compiler */
 #define __func__ __FUNCTION__
 #define __attribute__(x)
 #endif