[CMAKE] Don't install secp256k1 by default unless it is standalone

Summary:
It is no longer installed via ninja install but it is still possible
to install it with ninja install-secp256k1 as needed.

Test Plan:
On Bitcoin ABC project, check it is no longer installed:

ninja install

And install it via:

ninja install-secp256k1

On secp256k1 standalone library, check the lib is installed as
expected.:

ninja install

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7240