Make sure we type the cast properly.

http://bugs.gentoo.org/show_bug.cgi?id=63773

--- src/story_screen.cxx
+++ src/story_screen.cxx
@@ -142,7 +142,7 @@
 
   if (!page_displayed_completly)
     {
-      unsigned int len = static_cast<unsigned int>(20.0f * time_passed);
+      size_t len = static_cast<size_t>(20.0f * time_passed);
       display_text = current_page.text.substr(0, Math::min(current_page.text.length(), len));
 
       if (current_page.text.length() < len)