net: shell: stacks: Print config option needed for full info

This is similar to how few commands already behave if they can
provide additional info to the user if particular config options
are enabled.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
diff --git a/subsys/net/ip/net_shell.c b/subsys/net/ip/net_shell.c
index d25210c..f038f8c 100644
--- a/subsys/net/ip/net_shell.c
+++ b/subsys/net/ip/net_shell.c
@@ -2168,6 +2168,8 @@
 	       CONFIG_SYSTEM_WORKQUEUE_STACK_SIZE, unused,
 	       CONFIG_SYSTEM_WORKQUEUE_STACK_SIZE - unused,
 	       CONFIG_SYSTEM_WORKQUEUE_STACK_SIZE, pcnt);
+#else
+	printk("Enable CONFIG_INIT_STACKS to see usage information.\n");
 #endif
 
 	return 0;