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

[David Bremner]

Jani Nikula <jani at nikula.org> writes:

[...]

> 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.

pushed,

d