bpf: fix build without bpf_syscall

wrap bpf_stats_enabled sysctl with #ifdef

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 492ecee8 ("bpf: enable program stats")
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>