drivers: hv: Mark the function hv_synic_free_cpu() as static in hv.c

This patch marks the function hv_synic_free_cpu() as static in hv.c
because it is not used outside this file.

Thus, it also eliminates the following warning in hv.c:
drivers/hv/hv.c:304:6: warning: no previous prototype for ‘hv_synic_free_cpu’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>