libbpf: Fix spelling mistake "kallasyms" -> "kallsyms"

There is a spelling mistake in a pr_warn message. Fix it.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200623084207.149253-1-colin.king@canonical.com