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

drivers/hv/hv_balloon.c

@@ -545,7 +545,7 @@ static void post_status(struct hv_dynmem_device *dm)
 
 
 
-void free_balloon_pages(struct hv_dynmem_device *dm,
+static void free_balloon_pages(struct hv_dynmem_device *dm,
 			 union dm_mem_page_range *range_array)
 {
 	int num_pages = range_array->finfo.page_cnt;