w64: Fix time conversion for some versions of MinGW-w64

tb.time is a time value, but not necessarily of the same size as time_t:
while time_t is 64 bit for w64, tb.time still is 32 bit only.

Therefore we need en explicit conversion.
Signed-off-by: NStefan Weil <sw@weilnetz.de>

savevm.c

@@ -2106,7 +2106,8 @@ void do_savevm(Monitor *mon, const QDict *qdict)
         }
     } else {
 #ifdef _WIN32
-        ptm = localtime(&tb.time);
+        time_t t = tb.time;
+        ptm = localtime(&t);
         strftime(sn->name, sizeof(sn->name), "vm-%Y%m%d%H%M%S", ptm);
 #else
         /* cast below needed for OpenBSD where tv_sec is still 'long' */