Stats: don't include glib.h in header

Use forward declaration instead.

src/OtherCommands.cxx

@@ -47,6 +47,8 @@
 #include "StickerDatabase.hxx"
 #endif
 
+#include <glib.h>
+
 #include <assert.h>
 #include <string.h>
 

src/Stats.cxx

@@ -28,6 +28,8 @@
 #include "util/Error.hxx"
 #include "Log.hxx"
 
+#include <glib.h>
+
 struct stats stats;
 
 void stats_global_init(void)

src/Stats.hxx

@@ -20,9 +20,8 @@
 #ifndef MPD_STATS_HXX
 #define MPD_STATS_HXX
 
-#include <glib.h>
-
 class Client;
+typedef struct _GTimer GTimer;
 
 struct stats {
 	GTimer *timer;