common/fileutil: Convert namespace to Common::FS

Migrates a remaining common file over to the Common namespace, making it
consistent with the rest of common files.

This also allows for high-traffic FS related code to alias the
filesystem function namespace as

namespace FS = Common::FS;

for more concise typing.

src/yuzu/configuration/configure_debug.cpp

@@ -19,7 +19,8 @@ ConfigureDebug::ConfigureDebug(QWidget* parent) : QWidget(parent), ui(new Ui::Co
     SetConfiguration();
 
     connect(ui->open_log_button, &QPushButton::clicked, []() {
-        QString path = QString::fromStdString(FileUtil::GetUserPath(FileUtil::UserPath::LogDir));
+        const auto path =
+            QString::fromStdString(Common::FS::GetUserPath(Common::FS::UserPath::LogDir));
         QDesktopServices::openUrl(QUrl::fromLocalFile(path));
     });
 }