Fix build with CMake 3.8.

Before CMake 3.8, automoc'ed files were saved in the build directory.
With CMake 3.8, they're now saved into $targetName_autogen/include and exported
to $targetName's INCLUDE_DIRECTORIES variable.

mailserializertest.cpp #includes akonadi_serializer_mail.cpp
which #includes moc_akonadi_serializer_mail.cpp.

As a result, CMake had no way to guess where where the automoc'ed file is.