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/include/unistd.h b/libc/include/unistd.h
index 725cef5..ea6c8a1 100644
--- a/libc/include/unistd.h
+++ b/libc/include/unistd.h
@@ -33,13 +33,13 @@
 #include <sys/cdefs.h>
 #include <sys/types.h>
 #include <sys/select.h>
-#include <sys/sysconf.h>
 
 #include <bits/fcntl.h>
 #include <bits/getopt.h>
 #include <bits/ioctl.h>
 #include <bits/lockf.h>
 #include <bits/posix_limits.h>
+#include <bits/sysconf.h>
 
 __BEGIN_DECLS
 
@@ -214,8 +214,6 @@
 
 extern int acct(const char* __filepath);
 
-long sysconf(int __name);
-
 #if __ANDROID_API__ >= 21
 int getpagesize(void);
 #else