Qt6: Disable IR Sensor when compiling with Qt6

Gating the IR Sensor code behind a macro like so
`#if (QT_VERSION < QT_VERSION_CHECK(6, 0, 0)) && YUZU_USE_QT_MULTIMEDIA`

The YUZU_USE_QT_MULTIMEDIA flag is implemented in later commit

Also the locale fix in src/yuzu/main.cpp is now gated against Qt6,
as it causes compilation error