Mk/bsd.port.mk: Fix make describe-json

[fernape]

Summary

Properly escape DEPRECATED variable.

Otherwise, having something like this:

mail/p5-Email-Send$ make -VDEPRECATED
Send it's being officially marked "out of favor". Consider using mail/p5-Email-Sender for new projects

will create a broken JSON file.

Test Plan

Before applying the patch,

cd mail/p5-Email-Send && make describe-json | jq . will fail.

Apply patch and run the same command again.

[fernape]

Thanks!