Prepare to switch to the uapi <linux/signal.h>.

<time.h> didn't need to copy the cruft from <signal.h>, and
<signal.h> only needs the uid_t hack when it's not using
uapi headers.

pthread_exit.cpp should include what it uses.

Change-Id: I836c36abe0f0a781d41fc425b249d1c7686bb124
3 files changed