avutil/mem_internal: Don't include stdalign.h on MSVC

It's currently actually not used in MSVC builds, since
6e49b86996.

Older versions of MSVC (or, in particular, older versions of UCRT)
don't have stdalign.h; it's available since WinSDK 10.0.20348.0;
such a new enough version has been installed by default only since
MSVC 2022 17.4 and newer.

With this change, ffmpeg can still be built with MSVC 2019 16.8
(v19.28).

Signed-off-by: Martin Storsjö <martin@martin.st>

libavutil/mem_internal.h

@@ -24,7 +24,9 @@
 #include "config.h"
 
 #include <stdint.h>
+#ifndef _MSC_VER
 #include <stdalign.h>
+#endif
 
 #include "attributes.h"
 #include "macros.h"