Fix utility.cpp compilation error

[matdodgson]

Fix for utility.cpp compilation error.

Here's the error I got compiling on Ubuntu 24.10:

utility.cpp:266:19: error: no matching function for call to 'find'
  266 |         auto it = std::find(new_group_ptrs.begin(), new_group_ptrs.end(),
      |                   ^~~~~~~~~
/usr/lib/gcc/x86_64-linux-gnu/14/../../../../include/c++/14/bits/streambuf_iterator.h:435:5: note: candidate template ignored: could not match 'istreambuf_iterator' against '__normal_iterator'
  435 |     find(istreambuf_iterator<_CharT> __first,
      |     ^
1 error generated.