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/fcntl.h b/libc/include/fcntl.h
index 872a727..c49dbd0 100644
--- a/libc/include/fcntl.h
+++ b/libc/include/fcntl.h
@@ -37,6 +37,7 @@
 #include <linux/uio.h>
 
 #include <bits/fcntl.h>
+#include <bits/seek_constants.h>
 
 #if defined(__USE_GNU) || defined(__USE_BSD)
 #include <bits/lockf.h>