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/ncch_container.h

@@ -368,8 +368,8 @@ private:
     u32 partition = 0;
 
     std::string filepath;
-    FileUtil::IOFile file;
-    FileUtil::IOFile exefs_file;
+    Common::FS::IOFile file;
+    Common::FS::IOFile exefs_file;
 };
 
 } // namespace FileSys