Add header library target for eigen

Technically this should have already existed. I am making this a soong
build library so it is univerally accessible.

Bug: 36222421
Test: Built this and its depdencies.
Change-Id: I5124a7ee45512c18fe4f8cd86e7fa48db89ff74a
(cherry picked from commit 0462b8ffbe806de07b22c97fa777ea66c2dd7874)
1 file changed