libcrypt-util: use build-time check for crypt_preferred_method

After https://github.com/systemd/systemd/pull/16981 only the presence of crypt_gensalt_ra
is checked, but there are cases where that function is available but crypt_preferred_method
is not, and they are used in the same ifdef.
Add a check for the latter as well.

src/shared/libcrypt-util.c

@@ -40,7 +40,11 @@ int make_salt(char **ret) {
 
         e = secure_getenv("SYSTEMD_CRYPT_PREFIX");
         if (!e)
+#if HAVE_CRYPT_PREFERRED_METHOD
                 e = crypt_preferred_method();
+#else
+                e = "$6$";
+#endif
 
         log_debug("Generating salt for hash prefix: %s", e);