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

src/gui/channellist.cpp

@@ -47,6 +47,7 @@
 #include <driver/fade.h>
 #include <driver/display.h>
 #include <driver/radiotext.h>
+#include <driver/fontrenderer.h>
 
 #include <gui/color.h>
 #include <gui/color_custom.h>