Mark unaccent functions as STABLE, rather than defaulting to VOLATILE.

contrib/unaccent/unaccent.sql.in

@@ -6,12 +6,12 @@ SET search_path = public;
 CREATE OR REPLACE FUNCTION unaccent(regdictionary, text)
 	RETURNS text
 	AS 'MODULE_PATHNAME', 'unaccent_dict'
-	LANGUAGE C STRICT;
+	LANGUAGE C STABLE STRICT;
 
 CREATE OR REPLACE FUNCTION unaccent(text)
 	RETURNS text
 	AS 'MODULE_PATHNAME', 'unaccent_dict'
-	LANGUAGE C STRICT;
+	LANGUAGE C STABLE STRICT;
 
 CREATE OR REPLACE FUNCTION unaccent_init(internal)
 	RETURNS internal