x86/vpt: fix build with old gcc

x86/vpt: fix build with old gcc

I believe it was the XSA-336 fix (42fcdd42328f "x86/vpt: fix race when
migrating timers between vCPUs") which has unmasked a bogus
uninitialized variable warning. This is observable with gcc 4.3.4, but
only on 4.13 and older; it's hidden on newer versions apparently due to
the addition to _read_unlock() done by 12509bbeb9e3 ("rwlocks: call
preempt_disable() when taking a rwlock").

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>