sctp: enclose pre-init static function in between ifdef USE_SCTP

- it is not used otherwise and some compilers can throw warnings
- reported by Juha Heinanen

modules/sctp/sctp_mod.c

@@ -37,7 +37,9 @@
 MODULE_VERSION
 
 static int mod_init(void);
+#ifdef USE_SCTP
 static int sctp_mod_pre_init(void);
+#endif
 
 
 static cmd_export_t cmds[]={
@@ -122,6 +124,7 @@ static int mod_init(void)
 #endif /* USE_SCTP */
 }
 
+#ifdef USE_SCTP
 static int sctp_mod_pre_init(void)
 {
 	sctp_srapi_t api;
@@ -143,3 +146,4 @@ static int sctp_mod_pre_init(void)
 	}
 	return 0;
 }
+#endif