gnttab_usage_print() should be static

... as not being used or declared anywhere else.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -2802,7 +2802,7 @@ grant_table_destroy(
     d->grant_table = NULL;
 }
 
-void gnttab_usage_print(struct domain *rd)
+static void gnttab_usage_print(struct domain *rd)
 {
     int first = 1;
     grant_ref_t ref;