Update CMakeLists.txt: added Mutex.swift (#731)

Recently, Utilities/Mutex.swift was created but omitted in CMakeLists.txt

Author: dshan4585

Sources/ArgumentParser/CMakeLists.txt

@@ -43,6 +43,7 @@ add_library(ArgumentParser
   Usage/UsageGenerator.swift
 
   Utilities/CollectionExtensions.swift
+  Utilities/Mutex.swift
   Utilities/Platform.swift
   Utilities/SequenceExtensions.swift
   Utilities/StringExtensions.swift