client: fix a strdup memory leak

Memory leak found by valgrinding simple-shm client.
struct wl_global::interface is a strdup()'d string that was never freed.

Make a function for freeing a wl_global, and use it.

krh: Edit to name wl_global destructor wl_global_destroy.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>