make: Define dummy target for scripts/Makefile.version

At moment built-in .SUFFIXES variable is not cleaned
which slows down the building procedure. Add a dummy
rule thus the make won't try to lookup rules for
scripts/Makefile.version (this happens when last
resort rule is used).

Note that this is rather a stub for future convenience
which doesn't affect much current build procedure.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>