Correct relative system font load

src/crypts-and-corpses.lisp

   (terminal-open)
   (terminal-set "window: size=99x40, title='Crypts and Corpses'")
   (terminal-set (format nil "font: ~SDejaVuSansMono.ttf, size=16"
-                        (asdf:system-relative-pathname :crypts-and-corpses "fonts/")))
+                        (namestring (asdf:system-relative-pathname :crypts-and-corpses "fonts/"))))
   (intro-message)
   (main-loop)
   (terminal-close))