swscale/swscale: add av_warn_unused_result to sws_init_context

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

libswscale/swscale.h

@@ -150,6 +150,7 @@ struct SwsContext *sws_alloc_context(void);
  * @return zero or positive value on success, a negative value on
  * error
  */
+av_warn_unused_result
 int sws_init_context(struct SwsContext *sws_context, SwsFilter *srcFilter, SwsFilter *dstFilter);
 
 /**