nmk: clean gcov files, too

When criu is compiled with GCOV=1, .gc* files created are never
cleaned. While 'git clean -dxf' does the job and is popular
among the criu devs, it still feels good to have good ol'
make clean doing the right thing.

Reviewed-by: Dmitry Safonov <dsafonov@virtuozzo.com>
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>