Consistently use 'op' in argument names.

This matches changes being made to regularize the man page terminology.

Change-Id: If949cf81a0a0a06f24a4a742e8bf45c53dbd0da1
diff --git a/libc/include/bits/fcntl.h b/libc/include/bits/fcntl.h
index 597aa6e..ee5a6e1 100644
--- a/libc/include/bits/fcntl.h
+++ b/libc/include/bits/fcntl.h
@@ -43,6 +43,6 @@
  *
  * The return value depends on the operation.
  */
-int fcntl(int __fd, int __cmd, ...);
+int fcntl(int __fd, int __op, ...);
 
 __END_DECLS