Avoid overflow when calculating time on systems with 32-bit time_t

[PanSzczerba]

Discovered when trying to run ethercrab's discover example with dev profile on raspberry pi 3B+. Generally this prevents the overflow, but it still leaves the code vunerable to Y2k38 problem. Unfortunetally libc crate doesn't yet provide the option to use 64 bit version of time_t on 32 bit systems, hence I submit only this minimal change.

Here's the full stacktrace for discover example:

thread 'main' panicked at src/std/unix/mod.rs:155:14:
attempt to multiply with overflow
stack backtrace:
   0: rust_begin_unwind
             at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/panicking.rs:652:5
   1: core::panicking::panic_fmt
             at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/core/src/panicking.rs:72:14
   2: core::panicking::panic_const::panic_const_mul_overflow
             at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/core/src/panicking.rs:180:21
   3: ethercrab::std::unix::ethercat_now
             at /project/src/std/unix/mod.rs:155:14
   4: core::ops::function::Fn::call
             at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/core/src/ops/function.rs:79:5
   5: ethercrab::dc::configure_dc::{{closure}}
             at /project/src/dc.rs:430:25
   6: ethercrab::maindevice::MainDevice::init::{{closure}}
             at /project/src/maindevice.rs:297:83
   7: ethercrab::maindevice::MainDevice::init_single_group::{{closure}}
             at /project/src/maindevice.rs:428:14
   8: discover::main::{{closure}}
             at /project/examples/discover.rs:46:14
   9: async_io::driver::block_on::{{closure}}
             at /cargo/registry/src/index.crates.io-6f17d22bba15001f/async-io-2.4.0/src/driver.rs:199:37
  10: std::thread::local::LocalKey<T>::try_with
             at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/thread/local.rs:286:12
  11: std::thread::local::LocalKey<T>::with
             at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/std/src/thread/local.rs:262:9
  12: async_io::driver::block_on
             at /cargo/registry/src/index.crates.io-6f17d22bba15001f/async-io-2.4.0/src/driver.rs:175:5
  13: discover::main
             at /project/examples/discover.rs:41:5
  14: core::ops::function::FnOnce::call_once
             at /rustc/129f3b9964af4d4a709d1383930ade12dfe7c081/library/core/src/ops/function.rs:250:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.