remove duplicate #includes

Prompted by Jakub Jelen
diff --git a/auth.h b/auth.h
index 04ad99f..becc672 100644
--- a/auth.h
+++ b/auth.h
@@ -234,7 +234,7 @@
 int	 sys_auth_passwd(struct ssh *, const char *);
 
 #if defined(KRB5) && !defined(HEIMDAL)
-#include <krb5.h>
 krb5_error_code ssh_krb5_cc_gen(krb5_context, krb5_ccache *);
 #endif
-#endif
+
+#endif /* AUTH_H */