x11/waybar: unbreak build with libc++ 17

In file included from ../src/modules/clock.cpp:1:
In file included from ../include/modules/clock.hpp:4:
In file included from ../include/util/date.hpp:29:
/usr/local/include/date/tz.h:235:8: error: use of overloaded operator '<<' is ambiguous (with operand types 'basic_ostream<char, char_traits<char>>' and 'const sys_seconds' (aka 'const time_point<std::chrono::system_clock, std::chrono::duration<long long, std::ratio<1, 1>>>'))

PR: 275651
Reported by: pkg-fallout