Fix height of button on SDDM login

[RobTheFiveNine]

This change fixes an issue with the SDDM login screen. The background of the button was set to an absolute height of 30, but the element itself was taller. As a result, there is an unequal amount of spacing between the top of the button and the bottom of the button, as can be seen in this screenshot prior to the final change (ignore that the button is max width, that was just me testing):

I have changed the background to instead match the height of the parent element, which results in the text being correctly vertically aligned in the center of the button:

Apologies for the camera shots instead of screenshots, I didn't have a VM to make the changes in, I did it on my host machine

[RobTheFiveNine]

@EliverLara could you take a look at this and #242 ? Would be good to get this one in particular merged in to fix this issue in the main distribution channel for KDE users