libchrome: Add test_future_internal.h

Install base/test/test_future_internal.h, since it's a prerequisite of
the previously enabled base/test/test_future.h.

BUG=none
TEST=emerge-$BOARD libchrome

Change-Id: Idbbe7958ed700ee28a65d8166923344ed2033280
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/libchrome/+/3808811
Tested-by: Maksim Ivanov <emaxx@chromium.org>
Reviewed-by: Qijiang Yūki Ishii <fqj@google.com>
Commit-Queue: Maksim Ivanov <emaxx@chromium.org>