Show t-shirt text at end of game inventory disclose

Restricting the text display only to the end of game disclose,
so it doesn't clutter the inventory during gameplay and so that
the readability of t-shirts is not given away.

include/extern.h

@@ -1824,6 +1824,7 @@ E long NDECL(random);
 /* ### read.c ### */
 
 E void FDECL(learnscroll, (struct obj *));
+E char *FDECL(tshirt_text, (struct obj *, char *));
 E int NDECL(doread);
 E boolean FDECL(is_chargeable, (struct obj *));
 E void FDECL(recharge, (struct obj *,int));