weston-log: rename a confusing parameter name in weston_log_subscription_printf()

In the function weston_log_subscription_printf() we have
a struct weston_log_subscription parameter that in the .c file
is named sub. In the .h, the same parameter is named scope.
This is confusing, since its type is not struct weston_log_scope,
but struct weston_log_subscription.

Rename the parameter in the .h to sub.

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

include/libweston/weston-log.h

@@ -89,7 +89,7 @@ weston_log_scope_printf(struct weston_log_scope *scope,
 			  const char *fmt, ...)
 			  __attribute__ ((format (printf, 2, 3)));
 void
-weston_log_subscription_printf(struct weston_log_subscription *scope,
+weston_log_subscription_printf(struct weston_log_subscription *sub,
 				const char *fmt, ...)
 			  __attribute__ ((format (printf, 2, 3)));
 void