Fix build. Libkleo is not a private dependency here.

In file included from /kde/build/5/kdepim/messageviewer/src/messageviewer/objecttreeparser.h:1:0,
                 from /kde/build/5/kdepim/messageviewer/src/globalsettings_messageviewer.h:8,
                 from /kde/src/5/kdepim/messageviewer/src/settings/messageviewersettings.h:27,
                 from /kde/src/5/kdepim/messageviewer/src/header/customheaderstyleplugin/customheaderstyle.cpp:22:
/kde/src/5/kdepim/messageviewer/src/viewer/objecttreeparser.h:42:33: fatal error: Libkleo/CryptoBackend: No such file or directory