y2038: fix socket.h header inclusion

Referencing the __kernel_long_t type caused some user space applications
to stop compiling when they had not already included linux/posix_types.h,
e.g.

s/multicast.c -o ext/sockets/multicast.lo
In file included from /builddir/build/BUILD/php-7.3.3/main/php.h:468,
                 from /builddir/build/BUILD/php-7.3.3/ext/sockets/sockets.c:27:
/builddir/build/BUILD/php-7.3.3/ext/sockets/sockets.c: In function 'zm_startup_sockets':
/builddir/build/BUILD/php-7.3.3/ext/sockets/sockets.c:776:40: error: '__kernel_long_t' undeclared (first use in this function)
  776 |  REGISTER_LONG_CONSTANT("SO_SNDTIMEO", SO_SNDTIMEO, CONST_CS | CONST_PERSISTENT);

It is safe to include that header here, since it only contains kernel
internal types that do not conflict with other user space types.

It's still possible that some related build failures remain, but those
are likely to be for code that is not already y2038 safe.

Reported-by: Laura Abbott <labbott@redhat.com>
Fixes: a9beb86a ("sock: Add SO_RCVTIMEO_NEW and SO_SNDTIMEO_NEW")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>