Move from MantidKernel/System.h to DllConfig.h

[peterfpeterson]

This is for PythonInterface, LiveData, and Nexus subpackages as part of #38457

To test:

This is a refactor to change #include and doesn't need more than the builds to pass.

This does not require release notes.

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[github-actions]

👋 Hi, @peterfpeterson,

Conflicts have been detected against the base branch. Please rebase your branch against the base branch.

[robertapplin]

Code changes look good, and builds are passing