[libc] Add "struct tm" declaration to <wchar.h> (#165795)
`<wchar.h>` should at least include the forward declaration of `struct tm`,
since it's needed for the `wcsftime` declaration (also, see
https://man7.org/linux/man-pages/man0/wchar.h.0p.html).
Even though we don't yet have `wcsftime`, some downstream users (notably
- libcxx) expects to see `struct tm` declaration there, to re-declare it under
`std` namespace:
https://github.com/llvm/llvm-project/blob/c46bfed1a484d30cd251a9a225649d74e3bf0af5/libcxx/include/cwchar#L135
So, add this type declaration to llvm-libc version of `wchar.h` now.
