compositor: Fold the log prototypes into compositor.h and drop log.h

We're trying to keep the API exposed by the core compositor in compositor.h
diff --git a/src/log.c b/src/log.c
index 0452e65..e5430d6 100644
--- a/src/log.c
+++ b/src/log.c
@@ -30,7 +30,7 @@
 #include <wayland-server.h>
 #include <wayland-util.h>
 
-#include "log.h"
+#include "compositor.h"
 
 static FILE *weston_logfile = NULL;