Update dbfwfilter build configuration

The filter now uses a more recent syntax to declare the function name
prefixes. This removes a build warning.

server/modules/filter/dbfwfilter/ruleparser.y

@@ -27,7 +27,7 @@
 %pure-parser
 
 /** Prefix all functions */
-%name-prefix="dbfw_yy"
+%name-prefix "dbfw_yy"
 
 /** The pure parser requires one extra parameter */
 %parse-param {void* scanner}