avutil/mem_internal: Don't use alignas for MSVC

MSVC messed up standard C features, again.

Signed-off-by: Zhao Zhili <zhilizhao@tencent.com>
Reviewed-by: James Almer <jamrial@gmail.com>

libavutil/mem_internal.h

@@ -78,6 +78,10 @@
     #define DECLARE_ALIGNED_T(n,t,v)    alignas(FFMIN(n, 16)) t v
     #define DECLARE_ASM_ALIGNED(n,t,v)  alignas(FFMIN(n, 16)) t av_used v
     #define DECLARE_ASM_CONST(n,t,v)    alignas(FFMIN(n, 16)) static const t av_used v
+#elif defined(_MSC_VER)
+    #define DECLARE_ALIGNED_T(n,t,v)    __declspec(align(n)) t v
+    #define DECLARE_ASM_ALIGNED(n,t,v)  __declspec(align(n)) t v
+    #define DECLARE_ASM_CONST(n,t,v)    __declspec(align(n)) static const t v
 #else
     #define DECLARE_ALIGNED_T(n,t,v)    alignas(n) t v
     #define DECLARE_ASM_ALIGNED(n,t,v)  alignas(n) t av_used v