sctp: fix unused lable warning

The proc file cleanup left a label possibly unused:

net/sctp/protocol.c: In function 'sctp_defaults_init':
net/sctp/protocol.c:1304:1: error: label 'err_init_proc' defined but not used [-Werror=unused-label]

This adds an #ifdef around it to match the respective 'goto'.

Fixes: d47d08c8 ("sctp: use proc_remove_subtree()")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>