kbuild: Add resolve_btfids clean to root clean target

The resolve_btfids tool is used during the kernel build,
so we should clean it on kernel's make clean.

Invoking the the resolve_btfids clean as part of root
'make clean'.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20210205124020.683286-5-jolsa@kernel.org