lguest: fix example launcher compilation for broken glibc headers.

Somehow a naked u16 slipped into the glibc headers on my Ubuntu machine
(i386 2.17-0ubuntu5), breaking compile:

	In file included from lguest.c:46:0:
	/usr/include/linux/virtio_net.h:188:2: error: unknown type name ‘u16’

We use the kernel-style types anyway, just define them before the includes.
Also remove the advice on adding missing headers: that no longer works.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>