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>