libtimeinstate: export cputimeinstate.h

Make the definitions in cputimeinstate.h available to this library's
users.

Test: build now succeeds after adding libtimeinstate as a dependency
in other projects
Bug: 138317993
Signed-off-by: Connor O'Brien <connoro@google.com>

Change-Id: I1148ab4e4f3492b723279a71f3c96c70e5c92f4e