d3d12va: Add a missing include for the declaration of ff_d3d12va_get_surface_index

This fixes the following build error:

src/libavcodec/d3d12va_decode.c:49:10: error: no previous prototype for function
 'ff_d3d12va_get_surface_index' [-Werror,-Wmissing-prototypes]
   49 | unsigned ff_d3d12va_get_surface_index(const AVCodecContext *avctx,
      |          ^

Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/d3d12va_decode.c

@@ -33,6 +33,7 @@
 #include "avcodec.h"
 #include "decode.h"
 #include "d3d12va_decode.h"
+#include "dxva2_internal.h"
 
 typedef struct HelperObjects {
     ID3D12CommandAllocator *command_allocator;