xwayland: Silence X wm debug logging

Much to verbose for default logging.  Use a wm_log() wrapper that we can
toggle on/off as needed.  Maybe we're ready for log domains now.
diff --git a/src/compositor.h b/src/compositor.h
index 4c746ca..774fb96 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -1073,6 +1073,10 @@
 void
 weston_log_file_close(void);
 int
+weston_vlog(const char *fmt, va_list ap);
+int
+weston_vlog_continue(const char *fmt, va_list ap);
+int
 weston_log(const char *fmt, ...)
 	__attribute__ ((format (printf, 1, 2)));
 int