[libcxx] Add a missing include for __enable_if_t (llvm#127016)

This is to fix compile error with explicit Clang modules like
```
../../third_party/libc++/src/include/__filesystem/path.h:80:26: error: declaration of '__enable_if_t' must be imported from module 'std_core.type_traits.enable_if' before it is required
   80 | template <class _ECharT, __enable_if_t<__can_convert_char<_ECharT>::value, int> = 0>
      |                          ^
../../third_party/libc++/src/include/__type_traits/enable_if.h:34:1: note: declaration here is not visible
   34 | using __enable_if_t _LIBCPP_NODEBUG = typename enable_if<_Bp, _Tp>::type;
      | ^
```

Author: atetubou

libcxx/include/__filesystem/path.h

@@ -18,6 +18,7 @@
 #include <__iterator/back_insert_iterator.h>
 #include <__iterator/iterator_traits.h>
 #include <__type_traits/decay.h>
+#include <__type_traits/enable_if.h>
 #include <__type_traits/is_pointer.h>
 #include <__type_traits/remove_const.h>
 #include <__type_traits/remove_pointer.h>