app-forensics/sleuthkit: unmask ewf USE flag for >=4.6.0
Starting with 4.6.0, we statically link sleuthkit to a locally
complied libewf since it is getting tree-cleaned (#547418). Mask the
USE flag in prior versions. See the commit message for the sleuthkit
4.6.0 bump for more details.
This partially reverts commit
37d9e41ab5c6fb2031aefdeb7af72a7354472031. The mask was put in place
without it being communicated to the maintainer.
Bug: https://bugs.gentoo.org/547418
Bug: https://bugs.gentoo.org/607968