util: Actually install thread_pool_types.h

util: Actually install thread_pool_types.h

The action portion of the rule for thread_pool_types.h was missing,
so the file was never installed. As that file is a dependency for the
whole of the util directory, it would result in util being completely
rebuild with every make invocation.

Add the missing installation rule, so we don't rebuild quite so often!

Change-Id: Idb201c239b6b37a25567cf231059ef05536fc258
Reviewed-on: http://gerrit.openafs.org/3921
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>