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/filebrowser.cpp

@@ -41,6 +41,7 @@
 #include <neutrino.h>
 #include <driver/display.h>
 #include <driver/screen_max.h>
+#include <driver/fontrenderer.h>
 
 #include <unistd.h>
 #include <stdio.h>