Makefile: Also install libcriu.so and headers

To link Open MPI against CRIU I need the header files installed.
Attached is a patch which adds the necessary Makefile rules to
install libcriu.so and the header file to compile against CRIU.

Signed-off-by: Adrian Reber <adrian@lisas.de>
Acked-by: Kir Kolyshkin <kir@openvz.org>
Acked-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>