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 <rob@landley.net>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/ftrace.c

@@ -172,6 +172,8 @@ int __init ftrace_dyn_arch_init(void)
 	return 0;
 }
 
+#ifdef CONFIG_MODULES
+
 static int __init ftrace_plt_init(void)
 {
 	unsigned int *ip;
@@ -190,6 +192,8 @@ static int __init ftrace_plt_init(void)
 }
 device_initcall(ftrace_plt_init);
 
+#endif /* CONFIG_MODULES */
+
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
 /*
  * Hook the return address and push it in the stack of return addresses