libx265: Properly handled dynamic linking with MSVC

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>

libavcodec/libx265.c

@@ -29,6 +29,10 @@
 #include "avcodec.h"
 #include "internal.h"
 
+#if defined(_MSC_VER)
+#define X265_API_IMPORTS 1
+#endif
+
 typedef struct libx265Context {
     const AVClass *class;