Fix compilation under CentOS 7 and Clang

Please check if the PR fulfills these requirements (please use '[x]' to check the checkboxes, or submit the PR and then click the checkboxes)

Does this PR already have an issue describing the problem ? If so, link to this issue using '#XXX' and skip the rest N/A

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...) Compilation issue

What is the current behavior? (You can also link to an open issue here) Under CentOS 7, with Clang, the build fails

What is the new behavior (if this is a feature change)? With these two fixes, the build works properly

Does this PR introduce a breaking change or deprecate an API? If yes, check the following:

[ ] The Breaking Change or Deprecated label has been added
[ ] The migration guide has been updated in the github wiki (What changes might users need to make in their application due to this PR?)

Other information:

(if any of the questions/checkboxes don't apply, please delete them entirely)

powsybl / powsybl-iidm4cpp