winstubs: include Misra/Std/Zstr.h for the Zstr typedef

The freestanding Windows-LLVM CI fails compiling `_WinStubs.c` with
`unknown type name 'Zstr'` at the `misra_getenv_stub` declaration.
The file uses `Zstr` (following the project convention -- no bare
`char *` / `const char *` outside the `_Generic` carve-out) but only
includes `Misra/Config.h`, which does not transitively pull in
`Zstr.h`. Standard-CI builds succeed because Sys.c is compiled first
in the same TU graph and earlier includes pull `Zstr.h` in via
something else; the freestanding LLVM build path doesn't get that
indirect include and fails.

Add the explicit `#include <Misra/Std/Zstr.h>`. The header is
header-only -- it brings in `Allocator.h`, `Vec/Type.h`, and
`Types.h`, all of which are typedef-only and libc-free, so the
freestanding constraint is preserved.

Author: brightprogrammer

Source/Misra/_WinStubs.c

@@ -24,6 +24,7 @@
 /// section (1) for the full design.
 
 #include <Misra/Config.h>
+#include <Misra/Std/Zstr.h>
 
 #if PLATFORM_WINDOWS