weston-log: rename the confusing function name weston_compositor_add_log_scope()

weston-log: rename the confusing function name weston_compositor_add_log_scope()

There's a function named weston_compositor_add_log_scope()
but it doesn't take a struct weston_compositor argument.

Rename it to weston_log_ctx_add_log_scope(), as
the log_scope is being added to a log_context.

Also, bump libweston_major to 9.

Signed-off-by: Leandro Ribeiro <leandrohr@riseup.net>