Guard against setPlaceholderText < QT 4.7

setPlaceholderText is unsupported in earlier QT versions.

See other checks of this kind:
https://github.com/mastercoin-MSC/mastercore/blob/mscore-0.0.9/src/qt/guiutil.cpp#L99-L101
https://github.com/mastercoin-MSC/mastercore/blob/mscore-0.0.9/src/qt/transactionview.cpp#L88-L90
https://github.com/mastercoin-MSC/mastercore/blob/mscore-0.0.9/src/qt/transactionview.cpp#L94-L96
...