Fix ABI break in kio

From the binary compatibility page "Do's and Don'ts":
https://community.kde.org/Policies/Binary_Compatibility_Issues_With_C%2B%2B#The_Do.27s_and_Don.27ts
You cannot...
[...]
For existing functions of any type:
[...]
inline it (this includes moving a member function's body to the class definition, even without the inline keyword).

REVIEW: 129032