x86, hyperv: Move a variable to avoid an unused variable warning

The variable hv_lapic_frequency causes an unused variable warning if
CONFIG_X86_LOCAL_APIC is disabled.  Since the variable is only used
inside a small if statement, move the declaration of that variable
into the if statement itself.

Cc: K. Y. Srinivasan <kys@microsoft.com>
Link: http://lkml.kernel.org/r/1381444224-3303-1-git-send-email-kys@microsoft.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>