config: Replace xstrdup with strdup in add_option()

All callers of add_option pass string literal as "key" argument
except one, where non-NULL condition is guarded by if().

Signed-off-by: Mikhail Gusarov <dottedmag@dottedmag.net>
Reviewed-by: Alan Coopersmith <alan.coopersmith@oracle.com>

config/config.c

@@ -133,7 +133,7 @@ add_option(InputOption **options, const char *key, const char *value)
     *options = calloc(sizeof(**options), 1);
     if (!*options) /* Yeesh. */
         return;
-    (*options)->key = xstrdup(key);
+    (*options)->key = strdup(key);
     (*options)->value = strdup(value);
     (*options)->next = NULL;
 }