[backport] scalar_impl.h: fix includes

Summary: group.h functions are not referenced. utils.h added as functions like VERIFY_CHECK are used.

This is a backport of libsecp256k1 PR640

Test Plan: ninja check-secp256k1

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5350