Directs pnmtopng stderr output to dev/null (issue 5973044)

[PhilEHolmes]

Reviewers: Julien Rioux, Graham Percival, dak,

Message:
Please review.

Description:
I'm directing this to dev null because there's no point in using a
logfile here.  The message from pnmtopng is:

pnmtopng: 256 colors found

and it displays this for greyscale images.  If this line fails, it will
be pngtopnm that will fail, and we will continue to get the error
messages from that.  I can see no circumstance when pnmtopng will fail,
and so we don't need to see its output.

Please review this at http://codereview.appspot.com/5973044/

Affected files:
  M Documentation/ly-examples/GNUmakefile


Index: Documentation/ly-examples/GNUmakefile
diff --git a/Documentation/ly-examples/GNUmakefile  
b/Documentation/ly-examples/GNUmakefile
index  
b4e4d2c0be13cbeceddfac602103a0fb93e0341a..ad1c84ae49d5a7df0d857c50e3282b99bf5d0b86  
100644
--- a/Documentation/ly-examples/GNUmakefile
+++ b/Documentation/ly-examples/GNUmakefile
@@ -18,4 +18,4 @@ $(outdir)/%.png: %.ly
        mv $(outdir)/$*.preview.png $@

 $(outdir)/%-small.png: $(outdir)/%.png
-       pngtopnm $< | pnmscale -w=600 | pnmtopng > $@
+       pngtopnm $< | pnmscale -w=600 | pnmtopng > $@ 2> /dev/null