mjpeg_decoder - fix for unused parameter.

TBR=kjellander@chromium.org
BUG=libyuv:678
TEST=None

Change-Id: I5206de2026b56f155531a76f94aeb6cb1b0cd6c9
Reviewed-on: https://chromium-review.googlesource.com/442090Reviewed-by: Frank Barchard <fbarchard@google.com>
Commit-Queue: Frank Barchard <fbarchard@google.com>

source/mjpeg_decoder.cc

@@ -431,7 +431,7 @@ void skip_input_data(j_decompress_ptr cinfo, long num_bytes) {  // NOLINT
 }
 
 void term_source(j_decompress_ptr cinfo) {
-  // Nothing to do.
+  (void)cinfo;  // Nothing to do.
 }
 
 #ifdef HAVE_SETJMP