wireguard: selftests: test using new 64-bit time_t

In case this helps expose bugs with the newer 64-bit time_t types, we do
our testing with the newer musl that supports this as well as
CONFIG_COMPAT_32BIT_TIME=n. This matters to us, since wireguard does in
fact deal with timestamps.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: David S. Miller <davem@davemloft.net>