net: diag: add missing declarations

The functions inet_diag_msg_common_fill and inet_diag_msg_attrs_fill
seem to have been missed from the include/linux/inet_diag.h header
file. Add them to fix the following warnings:

net/ipv4/inet_diag.c:69:6: warning: symbol 'inet_diag_msg_common_fill' was not declared. Should it be static?
net/ipv4/inet_diag.c:108:5: warning: symbol 'inet_diag_msg_attrs_fill' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>