<time.h>: Add timegm(), timelocal() and others.

Add timegm(), timelocal(), time2posix() and posix2time() to the
C library.

Change-Id: I34d5771ed83dd994870a5ca58a511d01898b1ffb
diff --git a/libc/include/time.h b/libc/include/time.h
index 5d1a0e2..4ad4f7b 100644
--- a/libc/include/time.h
+++ b/libc/include/time.h
@@ -128,6 +128,11 @@
 extern int  timer_gettime(timer_t timerid, struct itimerspec *value);
 extern int  timer_getoverrun(timer_t  timerid);
 
+extern time_t timelocal(struct tm *tm);
+extern time_t timegm(struct tm* tm);
+extern time_t time2posix(time_t ti);
+extern time_t posix2time(time_t ti);
+
 __END_DECLS
 
 #endif /* _TIME_H_ */