api tools: fix usage of stat by-name interface

Fixes: f6395eb6 ("hw-address: check if source file changed so cache needs update")
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

PVE/API2Tools.pm

@@ -4,6 +4,7 @@ use strict;
 use warnings;
 
 use Digest::MD5 qw(md5_hex);
+use File::stat;
 use Net::IP;
 use URI::Escape;
 use URI;