docs: virt: user_mode_linux.rst: update compiling instructions

Instead of pointing for a pre-2.4 and a seaparate patch,
update it to match current upstream, as UML was merged
a long time ago.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>