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

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

Reviewed By: philnik, #libc

Spies: libcxx-commits

Differential Revision: https://reviews.llvm.org/D141239

Author: JoeLoser

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

@@ -15,6 +15,7 @@
 #include <cstring>
 #include <cassert>
 #include <cstdint>
+#include <type_traits>
 
 #include "test_macros.h"