[CMAKE] Avoid rebuilding sec256k1

Summary:
Running multiple consecutive builds on any target that depends on
secp256k1 will trigger a rebuild of the library once every 2 calls.

The reason for this is that the gen_context utility is producing the
ecmult_static_context.h file under the src directory, but the custom
command expects the output in the current binary directory which is a
level above. The custom command ends up running everytime to try
regenerating the output, which in turns cause secp256k1 to rebuild as
the header is modified.

Test Plan:

ninja bitcoind

Then:

time for i in {1..10}; do ninja bitcoind; done

Enjoy :)

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D4587