selftest/bpf: Fix spelling mistake "SIGALARM" -> "SIGALRM"

There is a spelling mistake in an error message, fix it.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20200514121529.259668-1-colin.king@canonical.com