if prototype is declared static make function static to match

Today, a compiler was encountered that considered it an error
to have the prototype declared static and the function body
not

src/display.c

@@ -897,7 +897,7 @@ xchar x, y;
     }
 }
 
-int
+STATIC_OVL int
 tether_glyph(x, y)
 int x, y;
 {