libaktualizr: Fix bug when building out-of-tree app
libaktualizr: Fix bug when building out-of-tree app

I've been experimenting with building aktualizr-lite out-of-tree with
aktualizr as a git submodule as described in the docs. I was getting the
following error without this fix:

../aktualizr/src/libaktualizr/utilities/types.cc:62:37: error: variable 'std::istringstream jsonss' has initializer but incomplete type
   62 |   std::istringstream jsonss(json_str);
      |                                     ^

Signed-off-by: Andy Doan <e3579b1e47f273529f0f929453e939a68ede9fd1@foundries.io>

advancedtelematic/aktualizr,advancedtelematic/aktualizr,advancedtelematic/aktualizr,advancedtelematic/aktualizr