Since commit c895f6f7 ("bpf: correct broken uapi for
BPF_PROG_TYPE_PERF_EVENT program type"), all architectures
(except um) are required to have bpf_perf_event.h in uapi/asm.

Add it to mandatory-y so "make headers_install" can check it.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>