POSIX says <fcntl.h> makes SEEK_* visible.

http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/fcntl.h.html

We already had two copies of these constants, so time to factor them out
into bits/.

Change-Id: I213c0d2372e41463593f878d338002b8eaa918cd
diff --git a/libc/include/unistd.h b/libc/include/unistd.h
index 86949de..ac7d4c2 100644
--- a/libc/include/unistd.h
+++ b/libc/include/unistd.h
@@ -39,6 +39,7 @@
 #include <bits/ioctl.h>
 #include <bits/lockf.h>
 #include <bits/posix_limits.h>
+#include <bits/seek_constants.h>
 #include <bits/sysconf.h>
 
 __BEGIN_DECLS
@@ -52,10 +53,6 @@
 #define W_OK 2
 #define R_OK 4
 
-#define SEEK_SET 0
-#define SEEK_CUR 1
-#define SEEK_END 2
-
 #define _PC_FILESIZEBITS 0
 #define _PC_LINK_MAX 1
 #define _PC_MAX_CANON 2