libweave: BASE_EXPORTS for smart pointers

This types is likely be used externally.

BUG=none
TEST=none

Change-Id: Icda79d4a3bf8264e8d2f0bcd2cd003c2d8fd78cf
Reviewed-on: https://chromium-review.googlesource.com/295415
Tested-by: Vitaly Buka <vitalybuka@chromium.org>
Reviewed-by: Vitaly Buka <vitalybuka@chromium.org>
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Commit-Queue: Vitaly Buka <vitalybuka@chromium.org>