net_sched: fix a compile warning in act_ife

Apparently ife_meta_id2name() is only called when
CONFIG_MODULES is defined.

This fixes:

net/sched/act_ife.c:251:20: warning: ‘ife_meta_id2name’ defined but not used [-Wunused-function]
 static const char *ife_meta_id2name(u32 metaid)
                    ^~~~~~~~~~~~~~~~

Fixes: d3f24ba8 ("net sched actions: fix module auto-loading")
Cc: Roman Mashak <mrv@mojatatu.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>