Add new EVENT keyword to unreserved_keyword production.

Once again, somebody who ought to know better forgot this.  We really
need some automated cross-check on the keyword-list productions, I think.
Per report from Brian Weaver.

src/backend/parser/gram.y

@@ -12444,6 +12444,7 @@ unreserved_keyword:
 			| ENCRYPTED
 			| ENUM_P
 			| ESCAPE
+			| EVENT
 			| EXCLUDE
 			| EXCLUDING
 			| EXCLUSIVE