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