pr: Make prdate() more obviously correct

Using ctime() as pr()'s first argument is safe, because its value
never contains '%'.  Clean it up anyway, so we can enable
-Wformat-security.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>

src/lib/subs/pr.c

@@ -432,7 +432,7 @@ prdate(void)
     time_t now;
 
     (void)time(&now);
-    pr(ctime(&now));
+    pr("%s", ctime(&now));
 }
 
 /*