NANDContentLoader: Make CNANDContentData's Get function return by non-const value

const specifiers like this are practically pointless and can inhibit move construction.

Source/Core/DiscIO/NANDContentLoader.cpp

@@ -107,7 +107,7 @@ void CNANDContentDataFile::Open()
 {
   EnsureOpen();
 }
-const std::vector<u8> CNANDContentDataFile::Get()
+std::vector<u8> CNANDContentDataFile::Get()
 {
   std::vector<u8> result;
   EnsureOpen();