hv: hv_balloon: mark a function static

This resolves the following sparse warning:

drivers/hv/hv_balloon.c:548:6: sparse: symbol 'free_balloon_pages' was not declared. Should it be static?

Reported-by: Xie ChanglongX <changlongx.xie@intel.com>
Cc: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>