demos/gtk-demo/gtkfishbowl.c: Include gtk/fallback-c89.c

...in place of math.h, as we are using round(), which is possibly not
provided by the compiler since we don't require a C99 compiler in
GTK+-3.x and gtk/fallback-c89.c does include math.h anyways.

demos/gtk-demo/gtkfishbowl.c

@@ -19,7 +19,7 @@
 
 #include "gtkfishbowl.h"
 
-#include <math.h>
+#include "gtk/fallback-c89.c"
 
 typedef struct _GtkFishbowlPrivate       GtkFishbowlPrivate;
 typedef struct _GtkFishbowlChild         GtkFishbowlChild;