Revert "Remove unecessary header in amount.h"
Summary:
This reverts commit ffd121e24b842daee2a11e64eb1c750d80599ecf.
This header is neccessary to compile on MacOS X.
Test Plan:
make check
Reviewers: #bitcoin_abc, jasonbcox
Reviewed By: #bitcoin_abc, jasonbcox
Subscribers: teamcity
Differential Revision: https://reviews.bitcoinabc.org/D1627