avutil/internal: add av_warn_unused_result to avpriv_open

The open syscall can obviously fail, and its return code needs to be
checked.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>

libavutil/internal.h

@@ -259,6 +259,7 @@ void avpriv_request_sample(void *avc,
 /**
  * A wrapper for open() setting O_CLOEXEC.
  */
+av_warn_unused_result
 int avpriv_open(const char *filename, int flags, ...);
 
 int avpriv_set_systematic_pal2(uint32_t pal[256], enum AVPixelFormat pix_fmt);