fix build failure with glib 2.67

Based on a patch from Michael J Gruber [1].  As of glib 2.67 (more
specifically [2]), including "gmime-extra.h" inside an extern "C"
block causes build failures, because glib is using C++ features.

Observing that "gmime-extra.h" is no longer needed in
notmuch-private.h, which can simply delete that include, but
we have to correspondingly move the includes which might include
it (in particular crypto.h) out of the extern "C" block also.

This seems less fragile than only moving gmime-extra, and relying on
preprocessor sentinels to keep the deeper includes from happening.

Move to the include to the outside of the extern block.

[1]: id:aee618a3d41f7889a7449aa16893e992325a909a.1613055071.git.git@grubix.eu
[2]: https://gitlab.gnome.org/GNOME/glib/-/merge_requests/1715