Get rid of warnings when compiled with -Wformat-security

This is a backport of a commit from upstream libxml2, made on
2009-06-04 09:22:45.  See the upstream commit message at
http://git.gnome.org/browse/libxml2/commit/?id=bccae2d21038967f5d83109f27b925bcd4795602

Change-Id: Idc338d78357a03f5a6559a8ddf30ded2aa51a498
8 files changed