Fix lockstat:::thread-spin dtrace probe with LOCK_PROFILING

The spinning start time is missing from the calculation due to a
misplaced #endif. Return the #endif where it's supposed to be.

Submitted by: Alexander Alexeev <aalexeev@isilon.com>
Reviewed by: bdrewery, mjg
MFC after: 1 week
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D31384