qt: Use "fusion" style on macOS Big Sur with old Qt
qt: Use "fusion" style on macOS Big Sur with old Qt

The "macintosh" style is broken on macOS Big Sur at least for Qt 5.9.8.

Github-Pull: #bitcoin-core/gui#177
Rebased-From: 4e1154dfd128cbada65e9ea08ee274cdeafc4c53

rnicoll/dogecoin,qtumproject/qtum,rnicoll/dogecoin,litecoin-project/litecoin,rnicoll/dogecoin,litecoin-project/litecoin,litecoin-project/litecoin,qtumproject/qtum,rnicoll/dogecoin,litecoin-project/litecoin,qtumproject/qtum,litecoin-project/litecoin,rnicoll/dogecoin,qtumproject/qtum,qtumproject/qtum,qtumproject/qtum,qtumproject/qtum,litecoin-project/litecoin