avdevice/internal: add av_warn_unused_result

This does not trigger any warnings but adds robustness.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>

libavdevice/internal.h

@@ -21,6 +21,7 @@
 
 #include "libavformat/avformat.h"
 
+av_warn_unused_result
 int ff_alloc_input_device_context(struct AVFormatContext **avctx, struct AVInputFormat *iformat,
                                   const char *format);