watchman: Fix add_glob parameter type
watchman: Fix add_glob parameter type

Summary:
The only caller of add_glob passes the `w_string` returned from `json_to_w_string`.

The `w_string` type has an implicit conversion to `w_string_t*` which allows it
to be passed in to `add_glob` without a cast.  The implicit conversion is
undesirable in this case because it makes it more awkward to use some of the
newer methods that are present on the `w_string` type.

This diff fixes up the method to accept to `w_string` reference.

It has no functional change.

Reviewed By: fanzeyi

Differential Revision: D17679772

fbshipit-source-id: ffb83bf984c1d05bd28556b623e3b19c54d34fa7

facebook/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,nodakai/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman