r307082 added the TCP_HHOOK kernel option and made some existing code only
compile when that option is configured. In tcp_destroy(), the error
variable is now only used in code enclosed in an '#ifdef TCP_HHOOK' block.
This broke the build for VNET images.

Enclose the error variable itself in an #ifdef block.

Submitted by: Shawn Webb <shawn.webb at hardenedbsd.org>
Reported by: Shawn Webb <shawn.webb at hardenedbsd.org>
PointyHat to: jtl