include: cache copy of include/linux/socket.h

This fixes compilation in Debian Squeeze:

  CC     attr.lo
In file included from ../include/libmnl/libmnl.h:9,
                 from socket.c:10:
../include/linux/netlink.h:35: error: expected specifier-qualifier-list before ‘__kernel_sa_family_t’
In file included from ../include/libmnl/libmnl.h:9,
                 from callback.c:11:
../include/linux/netlink.h:35: error: expected specifier-qualifier-list before ‘__kernel_sa_family_t’
socket.c: In function ‘mnl_socket_get_portid’:
socket.c:102: error: ‘const struct sockaddr_nl’ has no member named ‘nl_pid’
socket.c: In function ‘mnl_socket_bind’:
socket.c:146: error: ‘struct sockaddr_nl’ has no member named ‘nl_family’
socket.c:147: error: ‘struct sockaddr_nl’ has no member named ‘nl_groups’
socket.c:148: error: ‘struct sockaddr_nl’ has no member named ‘nl_pid’
socket.c:163: error: ‘struct sockaddr_nl’ has no member named ‘nl_family’
socket.c: In function ‘mnl_socket_sendto’:
socket.c:184: error: unknown field ‘nl_family’ specified in initializer
socket.c:185: warning: excess elements in struct initializer
socket.c:185: warning: (near initialization for ‘snl’)

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>