kselftest: add kselftest-clean rule

We use

    $make TARGETS="size timers" kselftest

to build and run selftests. but there is no rule
for us to clean the kselftest generated files.

This patch add the rules, for example:

	$ make TARGETS="size timers" kselftest-clean

can clean all kselftest generated files.

Signed-off-by: Wang Long <long.wanglong@huawei.com>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Michal Marek <mmarek@suse.com>