Revert "Figure out the GTK version instead of using a hardcoded one"

Changing build requirements in a stable release is a great way to annoy
distributions and violates the principle of least surprise.

CCMAIL: rohan@garg.io
CCMAIL: plasma-devel@kde.or

This reverts commit d25084c791d82850fb9ea0f3a5ad178e28991e4b.