...and move tags/cscope removal to it.

'make clean' is traditionally used to remove all
compiler-generated stuff in order to do a clean rebuild.

'make distclean' cleans more, it is used to prepare sources
for 'make dist' (creating a source tarball).

It is unfortunate to find out you have to run 'make tags cscope'
every time after 'make clean'.
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>