selftests/bpf: Re-generate vmlinux.h and BPF skeletons if bpftool changed

Trigger vmlinux.h and BPF skeletons re-generation if detected that bpftool was
re-compiled. Otherwise full `make clean` is required to get updated skeletons,
if bpftool is modified.

Fixes: acbd0620 ("selftests/bpf: Add vmlinux.h selftest exercising tracing of syscalls")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210318194036.3521577-11-andrii@kernel.org