etc/mtree/BSD.include.dist: Remove libmilter-dev

[manu]

Summary

When building with WITHOUT_SENDMAIL this result in a FreeBSD-libmilter-dev
package with only this directory and a dependacy on FreeBSD-libmilter which
doesn't exists.

Sponsored by: Beckhoff Automation GMbH & Co. KG

[ivy]

please include

Fixes: 436618a427b4 ("etc/mtree: Add package tags for /usr/include")

so we can undo these changes later once we have a better solution. otherwise, looks fine.

[manu]

In D54193#1238097, @ivy wrote:

please include

Fixes: 436618a427b4 ("etc/mtree: Add package tags for /usr/include")

so we can undo these changes later once we have a better solution. otherwise, looks fine.

Added locally.