sandbox: include sys/types.h for uid_t, gid_t, pid_t

In glibc these happen to get transitively included by string (I think?)
but POSIX says these types live in sys/types.h and in other libcs
(such as musl) a direct include of sys/types.h is needed.

Bug: 1380656
Change-Id: Ia5db2c02183c822789cf90fce58a6037e37fcb75
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3998080
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1066548}