Fix enabling of SpinBox for number of threads in Make settings

Summary:

• use toggled slot instead of clicked to allow enabling by config file

BUG: 381240

Test Plan:

• open "Configure Make Settings", select "Override number of jobs"
• close, reopen, then find the spinbox enabled

Reviewers: KDevelop, kfunk

Reviewed By: KDevelop, kfunk

Subscribers: kfunk, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D6371