Merge install target instructions for BerkeleyDB

Summary: Noticed this while setting up the build on my new laptop

Test Plan: read it

Reviewers: deadalnix, schancel, #bitcoin_abc, Mengerian

Reviewed By: deadalnix, #bitcoin_abc, Mengerian

Subscribers: Mengerian, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1517