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/stdio.h b/libc/include/stdio.h
index 097be3a..38021ef 100644
--- a/libc/include/stdio.h
+++ b/libc/include/stdio.h
@@ -47,6 +47,8 @@
 #define __need_NULL
 #include <stddef.h>
 
+#include <bits/seek_constants.h>
+
 __BEGIN_DECLS
 
 #if defined(__clang__)
@@ -106,10 +108,6 @@
 #define	L_tmpnam	1024	/* XXX must be == PATH_MAX */
 #define	TMP_MAX		308915776
 
-#define SEEK_SET 0
-#define SEEK_CUR 1
-#define SEEK_END 2
-
 /*
  * Functions defined in ANSI C standard.
  */