replace fontrenderer.h include with forward declaration

in order to flatten the build dependency tree further, include
fontrenderer.h directly where needed, in header files a forward
declaration is enough


Origin commit data
------------------
Commit: 2303d94300
Author: Stefan Seyfried <seife@tuxbox-git.slipkontur.de>
Date: 2017-02-05 (Sun, 05 Feb 2017)

src/gui/components/cc_frm_button.cpp

@@ -32,6 +32,7 @@
 #include <gui/color_custom.h>
 #include <system/debug.h>
 #include "cc_frm_button.h"
+#include <driver/fontrenderer.h>
 
 using namespace std;