nmtst: return const address from nmtst_inet6_from_string()

Signed-off-by: Thomas Haller <thaller@redhat.com>

include/nm-test-utils.h

@@ -119,7 +119,7 @@ nmtst_inet4_from_string (const char *str)
 	return addr;
 }
 
-inline static struct in6_addr *
+inline static const struct in6_addr *
 nmtst_inet6_from_string (const char *str)
 {
 	static struct in6_addr addr;