[libc++] Granularize <type_traits> includes in <iterator>

Reviewed By: Mordante, #libc

Spies: libcxx-commits

Differential Revision: https://reviews.llvm.org/D140621

NOKEYCHECK=True
GitOrigin-RevId: 430b397f6715d4d2f2569716c93fcd6292ace3dc
diff --git a/include/fstream b/include/fstream
index 0c09f21..41e95bf 100644
--- a/include/fstream
+++ b/include/fstream
@@ -1757,6 +1757,7 @@
 #  include <limits>
 #  include <new>
 #  include <stdexcept>
+#  include <type_traits>
 #endif
 
 #endif // _LIBCPP_FSTREAM