samples: hello_world_user: fail in compilation if userspace is not enabled

The sample depends on CONFIG_USERSPACE.

Signed-off-by: Wojciech Sipak <wsipak@antmicro.com>

samples/userspace/hello_world_user/src/main.c

@@ -8,6 +8,10 @@
 #include <stdio.h>
 #define USER_STACKSIZE	2048
 
+#ifndef CONFIG_USERSPACE
+#error This sample requires CONFIG_USERSPACE.
+#endif
+
 struct k_thread user_thread;
 K_THREAD_STACK_DEFINE(user_stack, USER_STACKSIZE);