s390/ftrace: fix compile for !MODULES

Fix this compile error if CONFIG_MODULES is disabled:

arch/s390/built-in.o: In function `ftrace_plt_init':
arch/s390/kernel/ftrace.o:(.init.text+0x34cc): undefined reference to `module_alloc'

Reported-by: Rob Landley <[e-mail]>
Signed-off-by: Heiko Carstens <[e-mail]>
Signed-off-by: Martin Schwidefsky <[e-mail]>

arch/s390/kernel/ftrace.c

···
 	return 0;
 }
 
+#ifdef CONFIG_MODULES
+
 static int __init ftrace_plt_init(void)
 {
 	unsigned int *ip;
···
 	return 0;
 }
 device_initcall(ftrace_plt_init);
+
+#endif /* CONFIG_MODULES */
 
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
 /*