[Backtracing] Fix JSON output when not giving backtraces for all threads.

I'd omitted the key from the dictionary by mistake.

rdar://121430255

Author: al45tair

stdlib/public/libexec/swift-backtrace/JSON.swift

@@ -272,6 +272,7 @@ extension SwiftBacktrace {
       }
       write("]")
     } else {
+      write(#", "crashedThread": "#)
       outputJSONThread(ndx: target.crashingThreadNdx,
                        thread: crashingThread)
     }