gdb/value.c: add several headers to the include list

Building GDB currently fails to build with libc++, because libc++ is
stricter about which headers "leak" entities they're not guaranteed
to support. The following headers have been added:

* `<iterator>`, to support `std::back_inserter`
* `<utility>`, to support `std::move` and `std::swap`
* `<vector>`, to support `std::vector`

Change-Id: Iaeb15057c5fbb43217df77ce34d4e54446dbcf3d