use rk_UNCONST to un-constify

git-svn-id: svn://svn.h5l.se/heimdal/trunk/heimdal@15428 ec53bebd-3082-4978-b11e-865c3cabbd6b

lib/sl/sl.c

@@ -288,7 +288,7 @@ static char *sl_readline(const char *prompt)
     old = signal(SIGINT, sl_sigint);
     if(setjmp(sl_jmp))
 	printf("\n");
-    s = readline((char*)prompt);
+    s = readline(rk_UNCONST(prompt));
     signal(SIGINT, old);
     return s;
 }