Trousers: fix missing include in trousers.h

BUG=b:147611033
TEST=build OK.

Change-Id: I6b519d6380fd5c9b92cd977c03b8994e032a08ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/trousers/+/1997520
Tested-by: Leo Lai <cylai@google.com>
Commit-Queue: Leo Lai <cylai@google.com>
Reviewed-by: Andrey Pronin <apronin@chromium.org>
Reviewed-by: Maksim Ivanov <emaxx@chromium.org>
1 file changed