do not use deprecated bison-keyword

%error-verbose has been deprecated since Bison 3.0, which was released in 2013. In Bison 3.3.1 which was recently released, this has started causing warnings. Let's update the code to do this in the modern way intead, to avoid cluttering the output needlessly.

Signed-off-by: Erik Faye-Lund erik.faye-lund@collabora.com