[Qt] Fix height of BitcoinAmountField

Since #4556 the amountfield is too big under qt5 for me.

Before:
![toobig](https://cloud.githubusercontent.com/assets/2814559/5059815/7b0429c2-6d2d-11e4-934c-504e591f7404.png)

After:
![toobig1](https://cloud.githubusercontent.com/assets/2814559/5059814/7b027f14-6d2d-11e4-84ac-c5e65e44d25f.png)