Fix CMake Error in generators/spectre/CMakeLists.txt

Summary:
When porting from add_library to okular_add_generator, 2f9246ae42ce
missed to remove MODULE for spectre, which is already implicitly added
as can be seen here.

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