[CMAKE] Fix missing inclusion when libbitcoinconsensus is disabled
[CMAKE] Fix missing inclusion when libbitcoinconsensus is disabled

Summary:
D4229 introduced the `BUILD_LIBBITCOINCONSENSUS` switch, and moved the
`include(InstallationHelper)` directive conditionally. This will prevent
installing other targets when `BUILD_LIBBITCOINCONSENSUS` is disabled.
This diff fixes the issue by moving the directive to avoid conditional
inclusion.

Test Plan:
  cmake -GNinja .. -DBUILD_LIBBITCOINCONSENSUS=OFF
  ninja install

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4341

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