Also track dependencies of native targets

Summary: Now we can have accurate dependency tracking for native targets when using ninja.

Test Plan:

ninja
ninja # Check we don't rebuild any native target
touch ../src/secp256k1/src/group.h
ninja # Check that the gen_context step triggers.

Verify that the make build is still going.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5641