Fixes an oversight causing the position of heads to be used for area triggers, instead of the feet, rendering the System Shocked achievement unobtainable.

Overkill fat fingered a CopMovement function override into the sentry gun movement code, making position checks based on where the head is at, instead of the feet. Simply restoring the original function from CopMovement fixes it.
This could potentially affect how some AI calculations are done, but because of how the override was placed in the sentry gun code, which has nothing to do with cop AI, this doesn't seem like it was intentional by overkill.