CI: add MSVC build and fix building with MSVC (#907)

Yess yessss let's continue supporting the worst compiler toolchain
😈

Fell out of the changes in #905

CI changes were claude because I cannot be arsed with yaml.

Can explain the individual code fixes if ya want any more context.

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Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/spice2x/util/nt_loader.h

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <windows.h>
 #include <winternl.h>
 
 #define LDR_DLL_NOTIFICATION_REASON_LOADED 1
@@ -43,4 +44,4 @@ NTSTATUS NTAPI LdrRegisterDllNotification(
 
 NTSTATUS NTAPI LdrUnregisterDllNotification(
     PVOID Cookie
-);
+);