condvar: Clean up condvar.h a bit

• Remove a typedef that has been unused for a long time.
• Remove a LOCORE guard. MI headers like condvar.h don't need such a guard in general.
• Move a forward declaration into the _KERNEL block.
• Add a types.h include to make the file self-contained.

Reviewed by: kib
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D43449