Move <sys/sysconf.h> to <bits/sysconf.h>.

Also remove unneeded declaration of sysconf in <unistd.h>.

Bug: http://b/28067717
Change-Id: Ife4692e48ca97480e422db0686c61133e1d619aa
diff --git a/libc/bionic/clock.cpp b/libc/bionic/clock.cpp
index 053e9e7..fda0708 100644
--- a/libc/bionic/clock.cpp
+++ b/libc/bionic/clock.cpp
@@ -26,9 +26,9 @@
  * SUCH DAMAGE.
  */
 
-#include <time.h>
-#include <sys/sysconf.h>
 #include <sys/times.h>
+#include <time.h>
+#include <unistd.h>
 
 #include "private/bionic_constants.h"