include: don't ship `linux/socket.h`

include: don't ship `linux/socket.h`

It needs to be paired with `sys/socket.h` (and we don't want to ship it
as it is bound to userland interface). On older libc, like Android's
bionic, `sys/socket.h` is almost empty and all the definitions,
including non-kernel related ones, are in `linux/socket.h`. If we ship
our version of `linux/socket.h`, we have an empty `linux/socket.h` and
an empty `sys/socket.h` and almost no definitions.

Since `linux/netlink.h` extract `__kernel_sa_family_t` from
`linux/socket.h` and we don't have it anymore, we just replace its
definition by `unsigned short`. Being a Linux kernel interface, it is
quite unlikely to change anytime soon.