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/applets/profile_select.cpp

@@ -26,7 +26,7 @@ QString FormatUserEntryText(const QString& username, Common::UUID uuid) {
 }
 
 QString GetImagePath(Common::UUID uuid) {
-    const auto path = FileUtil::GetUserPath(FileUtil::UserPath::NANDDir) +
+    const auto path = Common::FS::GetUserPath(Common::FS::UserPath::NANDDir) +
                       "/system/save/8000000000000010/su/avators/" + uuid.FormatSwitch() + ".jpg";
     return QString::fromStdString(path);
 }