cli: mime node: fix compiler warning when building against gmime 2.4

commit d487ef9e58bcd193118f19f771d5ef3984616be5
Author: Jani Nikula <jani@nikula.org>
Date:   Sat Mar 30 15:53:16 2013 +0200

    cli: mime node: abstract decryption and signature verification

introduced a compiler warning, reported by Mark Walters, when building
against gmime 2.4:

mime-node.c:224:9: warning: assignment discards ‘const’ qualifier from
pointer target type [enabled by default]

Pass the non-const signature validity to the destructor to fix this.