devel/imake Add LICENSE

Add license information to devel/imake. [1]
As per discussions with portmgr, there is no need to add the different
varieties of MIT license, since they are so similar. Just note that this is
MIT licensed.

PR: 223255 [1] (based on)
Submitted by: Yasuhiro KIMURA [1]
Sponsored by: B3 Init (zeising)