xen_start_kernel() doesn't return.  Annotate it as such so objtool can
follow the code flow.
Signed-off-by: NJosh Poimboeuf <jpoimboe@redhat.com>
Link: https://lore.kernel.org/r/930deafa89256c60b180442df59a1bbae48f30ab.1611263462.git.jpoimboe@redhat.com