fix document generation

--- a/src/HOL/Auth/document/root.tex	Fri Feb 13 14:45:10 2009 -0800
+++ b/src/HOL/Auth/document/root.tex	Sat Feb 14 01:23:38 2009 -0800
@@ -1,5 +1,6 @@
 \documentclass[10pt,a4paper,twoside]{article}
 \usepackage{graphicx}
+\usepackage{amssymb}
 \usepackage[latin1]{inputenc}
 \usepackage{latexsym,theorem}
 \usepackage{isabelle,isabellesym}