Add blockindexworkcomparator.h to Makefile.am
Add blockindexworkcomparator.h to Makefile.am

Summary: It's required to do gitian build.

Test Plan: Do gitian build on OSX.

Reviewers: #bitcoin_abc, schancel, jasonbcox

Reviewed By: #bitcoin_abc, schancel, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D1640

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