compat: add linux/socket.h for __kernel_sa_family_t

compat: add linux/socket.h for __kernel_sa_family_t

The __kernel_sa_family_t type is not present on older systems (e.g.,
RHEL 6), and libnl3 will not built without it.  Copy
/usr/include/linux/socket.h from a RHEL7 system to
include/linux-private/linux/socket.h so that it will be found by the
build system.

Fortunately, this socket.h is small and self-contained; it contains
very little (and does not #include any other files) beyond necessary
type.

https://github.com/thom311/libnl/pull/107

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>
Signed-off-by: Thomas Haller <thaller@redhat.com>