Fix postfix plugin age to use ctime, not mtime (#3525)

plugins/inputs/postfix/stat_ctimespec.go

@@ -0,0 +1,16 @@
+// +build darwin freebsd
+
+package postfix
+
+import (
+	"syscall"
+	"time"
+)
+
+func statCTime(sys interface{}) time.Time {
+	stat, ok := sys.(*syscall.Stat_t)
+	if !ok {
+		return time.Time{}
+	}
+	return time.Unix(stat.Ctimespec.Unix())
+}