[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