[util] Print out --debug buffer to stderr

util/main-font-text.hh

diff --git a/util/main-font-text.hh b/util/main-font-text.hh
index 059dde4..55de567 100644
--- a/util/main-font-text.hh
+++ b/util/main-font-text.hh
@@ -59,7 +59,7 @@
 			      font,
 			      HB_BUFFER_SERIALIZE_FORMAT_TEXT,
 			      HB_BUFFER_SERIALIZE_FLAG_DEFAULT);
-  printf ("HB: buffer [%s]\n", buf);
+  fprintf (stderr, "HB: buffer [%s]\n", buf);
   return true;
 }