Fix ReactCxx xcodeproj

Summary:
The ReactCxx project was not updated when RCTMap was removed in 48f30ec, this fixes it. It was also missing the systemJSCWrapper.cpp file.

**Test plan**
Tested that UIExplorerCxx now builds

cc mkonicek
Closes #12919

Differential Revision: D4709498

Pulled By: mkonicek

fbshipit-source-id: 995a1a914ab0a3227b1219c575a84d136800ff19

Author: janicduplessis