Export kmime_codecs.h.

Summary:
As we export KMime/Codecs already, we should also export kmime_codecs.h.
Those functions to handle the codec encoded strings in kmime may be
useful outside of kmime so exporting those functions.

Test Plan:
Either export kmime_codecs.h or remove KMime/Codecs completly.
Both solutions will result in a successful abi-compliance-checker build.

Reviewers: vkrause, mlaurent

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D17831