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

libc/include/time.h

@@ -32,11 +32,6 @@
 #include <sys/cdefs.h>
 #include <sys/time.h>
 
-/* For struct sigevent. */
-#define __ARCH_SI_UID_T __kernel_uid32_t
-#include <asm/siginfo.h>
-#undef __ARCH_SI_UID_T
-
 __BEGIN_DECLS
 
 #define CLOCKS_PER_SEC 1000000
@@ -45,6 +40,8 @@ extern char* tzname[];
 extern int daylight;
 extern long int timezone;
 
+struct sigevent;
+
 struct tm {
   int tm_sec;
   int tm_min;