Drop libweave part from includes.

The extra prefix is cumbersome for packaging the library since that directory
part will contain a version number.

Change-Id: Iecaa0803185e5dd1d65d83fda79514351507b155
Reviewed-on: https://weave-review.googlesource.com/1160
Reviewed-by: Vitaly Buka <vitalybuka@google.com>
Reviewed-by: Alex Vakulenko <avakulenko@google.com>