[9P]: build fix with !CONFIG_SYSCTL

found via make randconfig build testing: 

 net/built-in.o: In function `init_p9':
 mod.c:(.init.text+0x3b39): undefined reference to `p9_sysctl_register'
 net/built-in.o: In function `exit_p9':
 mod.c:(.exit.text+0x36b): undefined reference to `p9_sysctl_unregister'
Signed-off-by: NIngo Molnar <mingo@elte.hu>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/9p/9p.h

@@ -412,6 +412,18 @@ int p9_idpool_check(int id, struct p9_idpool *p);
 
 int p9_error_init(void);
 int p9_errstr2errno(char *, int);
+
+#ifdef CONFIG_SYSCTL
 int __init p9_sysctl_register(void);
 void __exit p9_sysctl_unregister(void);
+#else
+static inline int p9_sysctl_register(void)
+{
+	return 0;
+}
+static inline void p9_sysctl_unregister(void)
+{
+}
+#endif
+
 #endif /* NET_9P_H */