doc: add qrencode to brew install instructions

Summary:
```
Build was missing qr code(qrencode libs) support
```

Backport of core PR12850
https://github.com/bitcoin/bitcoin/pull/12850/files

Depends on D3909.

Test Plan: Read it, test it.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D3911

doc/build-osx.md

@@ -23,7 +23,7 @@ Dependencies
 
 Install dependencies:
 
-    brew install automake berkeley-db libtool boost miniupnpc openssl pkg-config protobuf python qt libevent
+    brew install automake berkeley-db libtool boost miniupnpc openssl pkg-config protobuf python qt libevent qrencode
 
 See [dependencies.md](dependencies.md) for a complete overview.