dxva2: Prevent redefinition of _WIN32_WINNT

Don't redefine _WIN32_WINNT when targeting Win7/8
Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/dxva2_internal.h

@@ -23,7 +23,14 @@
 #ifndef AVCODEC_DXVA_INTERNAL_H
 #define AVCODEC_DXVA_INTERNAL_H
 
+#if defined(_WIN32_WINNT) && _WIN32_WINNT < 0x0600
+#undef _WIN32_WINNT
+#endif
+
+#if !defined(_WIN32_WINNT)
 #define _WIN32_WINNT 0x0600
+#endif
+
 #define COBJMACROS
 
 #include "config.h"