build: install -- Don't forget to init DESTDIR

When not set up it leaves empty making setup of
pycriu python library wrong. Define it as root.
Reported-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

Makefile.install

 #
 # Installation paths.
-DESTDIR		?=
+DESTDIR		?= /
 PREFIX		?= /usr/local
 SBINDIR		?= $(PREFIX)/sbin
 MANDIR		?= $(PREFIX)/share/man