libavutil: x86: Include stdlib.h before using _byteswap_ulong

When clang works in MSVC mode, it does have the _byteswap_ulong
builtin, but one has to include stdlib.h before using it.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavutil/x86/bswap.h

@@ -26,6 +26,7 @@
 
 #include <stdint.h>
 #if defined(_MSC_VER)
+#include <stdlib.h>
 #include <intrin.h>
 #endif
 #include "config.h"