Rename register.h header guard

Summary: Rename the header guard because _RPC_ indicates the package/directory that it resides in, so it should not be conjoined to the REGISTER_H portion.

Test Plan: make check

Reviewers: deadalnix, schancel, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1827