[DOC] Fix missing dependency in build-osx instructions

Summary:
librsvg is needed to generate the app image, add it to the
dependencies.

Test Plan: Follow the build-osx instructions on a fresh macos 10.15.6 installation.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7448