dxva2: include dxva.h if found

Apparently, some build environments require dxva.h even for dxva2,
while others lack this header entirely.  Including it conditionally
allows building in both cases.
Signed-off-by: Martin Storsjö <martin@martin.st>

configure

@@ -1071,6 +1071,7 @@ HAVE_LIST="
     dlfcn_h
     dlopen
     dos_paths
+    dxva_h
     ebp_available
     ebx_available
     exp2
@@ -2870,6 +2871,7 @@ check_func_headers windows.h Sleep
 check_func_headers windows.h VirtualAlloc
 
 check_header dlfcn.h
+check_header dxva.h
 check_header dxva2api.h
 check_header malloc.h
 check_header poll.h

libavcodec/dxva2_internal.h

@@ -25,7 +25,14 @@
 
 #define _WIN32_WINNT 0x0600
 #define COBJMACROS
+
+#include "config.h"
+
 #include "dxva2.h"
+#if HAVE_DXVA_H
+#include <dxva.h>
+#endif
+
 #include "avcodec.h"
 #include "mpegvideo.h"