minui: Export minui/minui.h.

For libminui static and shared libraries.

Test: build
Change-Id: Ib30dc5e2ef4a3c8b3ca3a0cec68cb65e229a0c16
diff --git a/minui/graphics.h b/minui/graphics.h
index 52968eb..1eaafc7 100644
--- a/minui/graphics.h
+++ b/minui/graphics.h
@@ -17,7 +17,7 @@
 #ifndef _GRAPHICS_H_
 #define _GRAPHICS_H_
 
-#include "minui.h"
+#include "minui/minui.h"
 
 // TODO: lose the function pointers.
 struct minui_backend {