Export definition of math functions

Need this to use it in Typed OM spec

Author: nainar

css-values-4/Overview.bs

@@ -1586,7 +1586,7 @@ Functional Notations</h2>
 <h3 id="calc-notation">
 Mathematical Expressions: ''calc()'', ''min()'', and ''max()''</h3>
 
-	The <dfn>math functions</dfn>,
+	The <dfn export>math functions</dfn>,
 	<dfn>calc()</dfn>, <dfn>min()</dfn>, and <dfn>max()</dfn>,
 	allow mathematical expressions
 	with addition (''+''), subtraction (''-''), multiplication (''*''), and division (''/'')