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

plugins/inputs/postfix/stat_ctim.go

@@ -0,0 +1,16 @@
+// +build dragonfly linux netbsd openbsd solaris
+
+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.Ctim.Unix())
+}