Fix missing exports with MSVC 2019

Fixes the Kaidan build with MSVC.

Weirdly enough, MSVC got confused on finding the matching overload for <<, so I replaced it with push_back().

PR check list:

[x] Document your code
[x] Add \since QXmpp 1.X, QXMPP_EXPORT
[x] Fix doxygen warnings (see log when building with -DBUILD_DOCUMENTATION=ON)
[x] Update doc/doap.xml
[x] Add unit tests
[ ] Format the code: Run clang-format -i src/<edited-file(s)> tests/<edited-file(s)>

qxmpp-project / qxmpp