hwcontext_vulkan: guard unistd.h include

win32 typically doesn't have unistd.h, so always including it will break
MSVC builds. The usage of those POSIX functions are already guarded by
_WIN32, so use that to guard unistd.h include as well.

(cherry picked from commit 185871fdd3)

libavutil/hwcontext_vulkan.c

@@ -27,10 +27,10 @@
 #include "compat/w32dlfcn.h"
 #else
 #include <dlfcn.h>
+#include <unistd.h>
 #endif
 
 #include "thread.h"
-#include <unistd.h>
 
 #include "config.h"
 #include "pixdesc.h"