[build-presents] Enable SwiftPM in incremental OS X builds.

 - This is enabled for the package builder bots and the Linux, I think it is an
   oversight that it has been disabled on the OS X bots for all this time.

Author: ddunbar

utils/build-presets.ini

@@ -236,7 +236,7 @@ assertions
 
 # Build llbuild & swiftpm here
 llbuild
-#swiftpm
+swiftpm
 
 dash-dash