su: suppress "warning: unused variable 'user_buf'"

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/loginutils/su.c b/loginutils/su.c
index 5bec4bc..c31e7e7 100644
--- a/loginutils/su.c
+++ b/loginutils/su.c
@@ -42,7 +42,9 @@
 	struct passwd *pw;
 	uid_t cur_uid = getuid();
 	const char *tty;
+#if ENABLE_FEATURE_UTMP
 	char user_buf[64];
+#endif
 	const char *old_user;
 
 	flags = getopt32(argv, "mplc:s:", &opt_command, &opt_shell);