libheadtracking: Add missing header.

<cassert> was previously transitively included via media/Pose.h
and Eigen/Geometry, but newer versions of Eigen do not include
this header.

Bug: 281744537
Test: presubmit
Flag: EXEMPT no runtime behavior change
Change-Id: If666bcd8a366b5c9e221c8f49b05f0bdd2cc868d