avcodec/v4l2_context: Finish draining if V4L2_BUF_FLAG_LAST is set

V4L2 api can indicate that flushing of the capture buffers is completed
by setting the V4L2_BUF_FLAG_LAST flag.
Use guards because the flag was only defined in Linux v4.2.

Reference:
linux/Documentation/media/uapi/v4l/dev-decoder.rst

    "The client must continue to handle both queues independently,
     similarly to normal decode operation. This includes:
     ...
     - queuing and dequeuing CAPTURE buffers, until a buffer marked with
       the V4L2_BUF_FLAG_LAST flag is dequeued"

Reviewed-by: Ming Qian <ming.qian@nxp.com>
Signed-off-by: Andriy Gelman <andriy.gelman@gmail.com>

libavcodec/v4l2_context.c

@@ -412,6 +412,10 @@ dequeue:
                 ctx->done = 1;
                 return NULL;
             }
+#ifdef V4L2_BUF_FLAG_LAST
+            if (buf.flags & V4L2_BUF_FLAG_LAST)
+                ctx->done = 1;
+#endif
         }
 
         avbuf = &ctx->buffers[buf.index];