Mark is_steady as [[maybe_unused]].

Fixes error: unused variable 'is_steady' [-Werror,-Wunused-const-variable]

Author: StephanTLavavej

libcxx/test/std/time/time.clock/time.clock.utc/types.compile.pass.cpp

@@ -36,11 +36,11 @@
 #include "test_macros.h"
 
 // class utc_clock
-using rep                = std::chrono::utc_clock::rep;
-using period             = std::chrono::utc_clock::period;
-using duration           = std::chrono::utc_clock::duration;
-using time_point         = std::chrono::utc_clock::time_point;
-constexpr bool is_steady = std::chrono::utc_clock::is_steady;
+using rep                                 = std::chrono::utc_clock::rep;
+using period                              = std::chrono::utc_clock::period;
+using duration                            = std::chrono::utc_clock::duration;
+using time_point                          = std::chrono::utc_clock::time_point;
+[[maybe_unused]] constexpr bool is_steady = std::chrono::utc_clock::is_steady;
 
 // Tests the values. Some of them are implementation-defined.
 LIBCPP_STATIC_ASSERT(std::same_as<rep, std::chrono::system_clock::rep>);