namespaces: move get_mq() inside #ifdef CONFIG_SYSCTL

| ipc/mq_sysctl.c:26: warning: 'get_mq' defined but not used

Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>