net: ipv4: Remove extern udp_v4_early_demux()/tcp_v4_early_demux() in .c files

net: ipv4: Remove extern udp_v4_early_demux()/tcp_v4_early_demux() in .c files

Function udp_v4_early_demux() was already declared in 'include/net/udp.h',
no need to keep the extern in 'ip_input.c', which may produce the
following checkpatch warning:

  WARNING: externs should be avoided in .c files
  #45: FILE: net/ipv4/ip_input.c:322:
  +enum skb_drop_reason udp_v4_early_demux(struct sk_buff *skb);

Replace it by including 'net/udp.h'. Do the same for tcp_v4_early_demux().

Signed-off-by: Wang Liang <wangliang74@huawei.com>
Link: https://patch.msgid.link/20251025092637.1020960-1-wangliang74@huawei.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>