[libc++][test] Fix missing include in `endian.pass.cpp`

[jloser]

Summary

endian.pass.cpp uses std::is_enum and friends but doesn't include
<type_traits>. Add the missing include.

[philnik]

Thanks for fixing this!

[philnik]

I've landed this for you, since this fixes the trunk CI.

[jloser]

In D141239#4035827, @philnik wrote:

I've landed this for you, since this fixes the trunk CI.

Thanks, I appreciate it. Was just about to get to it this morning now. :)

libcxx/test/std/numerics/bit/bit.endian/endian.pass.cpp

 
 // enum class endian;
 // <bit>
 
 #include <bit>
 #include <cstring>
 #include <cassert>
 #include <cstdint>
+#include <type_traits>
 
 #include "test_macros.h"
 
 int main(int, char**) {
     static_assert(std::is_enum<std::endian>::value, "");
 
 // Check that E is a scoped enum by checking for conversions.
     typedef std::underlying_type<std::endian>::type UT;