Fix header guards using reserved identifiers

Summary:
Identifiers beginning with an underscore followed immediately by an
uppercase letter are reserved.

Backport of core PR11151

Test Plan:

make

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2237