Fix CMake Error in generators/spectre/CMakeLists.txt

Summary:
When porting from `add_library` to `okular_add_generator`, 2f9246ae
missed to remove `MODULE` for spectre, which is already implicitly added
as can be seen [here](https://phabricator.kde.org/source/kcoreaddons/browse/master/KF5CoreAddonsMacros.cmake;83623a46c9862ee37535de93427dec0b0c55e24d$139).

This causes the upcoming CMake 3.10 to print the error `Cannot find
source file`, which can be fixed by removing the superfluous `MODULE`.

BUG: 386176

Test Plan:
No error shown anymore with CMake 3.10.0-rc4. Still works with CMake
3.5.1.

Reviewers: #okular, ngraham

Reviewed By: ngraham

Tags: #okular

Differential Revision: https://phabricator.kde.org/D8612