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/widget/shellwindow.cpp

@@ -37,6 +37,7 @@
 #include <sys/wait.h>
 #include <driver/abstime.h>
 #include <driver/framebuffer.h>
+#include <driver/fontrenderer.h>
 #include <gui/widget/textbox.h>
 #include <stdio.h>
 #include <poll.h>