doc/Makefile: make sure CRIU_VERSION is defined

I just discovered that if I run make right from Documentation/ subdir,
CRIU_VERSION is not defined so the generated man page is lacking it.

Let's fix by conditionally including ../Makefile.versions.
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

Documentation/Makefile

@@ -28,6 +28,9 @@ check:
 		$$B --version > /dev/null || exit 1;	\
 	done
 
+ifeq ($(CRIU_VERSION),)
+ include ../Makefile.versions
+endif
 $(FOOTER): ../Makefile.versions
 	$(call msg-gen, $@)
 	$(Q) echo ":doctype:       manpage"		>  $@