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/core/file_sys/registered_cache.cpp

@@ -728,7 +728,7 @@ InstallResult RegisteredCache::RawInstallNCA(const NCA& nca, const VfsCopyFuncti
         LOG_WARNING(Loader, "Overwriting existing NCA...");
         VirtualDir c_dir;
         { c_dir = dir->GetFileRelative(path)->GetContainingDirectory(); }
-        c_dir->DeleteFile(FileUtil::GetFilename(path));
+        c_dir->DeleteFile(Common::FS::GetFilename(path));
     }
 
     auto out = dir->CreateFileRelative(path);