netfilter: ipset: fix compiler warnings "'hash_ip4_data_next' declared inline after being called"

Some gcc versions warn about prototypes without "inline" when the declaration
includes the "inline" keyword. The fix generates a false error message
"marked inline, but without a definition" with sparse below 0.4.2.
Signed-off-by: Chris Friesen <chris.friesen@genband.com>
Signed-off-by: Jozsef Kadlecsik <kadlec@blackhole.kfki.hu>
Signed-off-by: Patrick McHardy <kaber@trash.net>

include/linux/netfilter/ipset/ip_set_ahash.h

@@ -392,7 +392,7 @@ type_pf_resize(struct ip_set *set, bool retried)
 	return 0;
 }
 
-static void
+static inline void
 type_pf_data_next(struct ip_set_hash *h, const struct type_pf_elem *d);
 
 /* Add an element to a hash and update the internal counters when succeeded,