perf bpf: Clean up the generated/copied vmlinux.h

When building perf with BPF skels we either copy the minimalistic
tools/perf/util/bpf_skel/vmlinux/vmlinux.h or use bpftool to generate a
vmlinux from BTF, storing the result in $(SKEL_OUT)/vmlinux.h.

We need to remove that when doing a 'make -C tools/perf clean', fix it.

Fixes: b7a2d774 ("perf build: Add ability to build with a generated vmlinux.h")
Reviewed-by: Ian Rogers <irogers@google.com>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: James Clark <james.clark@arm.com>
Cc: Tiezhu Yang <yangtiezhu@loongson.cn>
Cc: Yang Jihong <yangjihong1@huawei.com>
Cc: bpf@vger.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/Zbz89KK5wHfZ82jv@x1