notmuch-emacs: avoiding deprecated message-default-charset

[Daniel Kahn Gillmor]

When i'm building notmuch in a debian environment with emacs
1:26.1+1-3.2, i notice the following deprecation warnings:

In notmuch-maildir-setup-message-for-saving:
emacs/notmuch-maildir-fcc.el:172:31:Warning: ‘message-default-charset’ is an
    obsolete variable (as of 26.1); The default charset comes from the
    language environment

I'm not enough of an emacs guru to know the right way to avoid this
warning, but i'm hoping that someonen else can take a stab at it, since
extraneous warnings make it easier to ignore real problems.

Sorry to provide a bug report with no proposed patch :/

         --dkg
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