Fix narrowing conversion warnings

[jagerman]

Fixes this warning:

libquic/src/internal.hpp: In function ‘timeval oxen::quic::loop_time_to_timeval(std::chrono::microseconds)’:
libquic/src/internal.hpp:42:34: warning: narrowing conversion of ‘std::chrono::operator/<long long int, std::ratio<1, 1000000>, long long int, std::ratio<1> >(t, std::literals::chrono_literals::operator""s<'1'>())’ from ‘time_t’ {aka ‘long long int’} to ‘long int’ [-Wnarrowing]
   42 |         return timeval{.tv_sec = static_cast<time_t>(t / 1s), .tv_usec = static_cast<suseconds_t>((t % 1s) / 1us)};
      |                                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~