gfio: fix discrepancies between graph header and implementation

Header defined a couple functions as returning void, but they really return int.

Signed-off-by: Stephen M. Cameron <stephenmcameron@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/graph.c b/graph.c
index d7bdbe5..1194510 100644
--- a/graph.c
+++ b/graph.c
@@ -29,6 +29,7 @@
 #include <gtk/gtk.h>
 
 #include "tickmarks.h"
+#include "graph.h"
 
 struct xyvalue {
 	double x, y;