SourceKit: fix a missing cmake dependency of swiftLang on Darwin

Author: eeckstein

tools/SourceKit/tools/swift-lang/CMakeLists.txt

@@ -19,6 +19,7 @@ add_swift_library(swiftSwiftLang SHARED
   UIDs.swift.gyb
 
   DEPENDS ${DEPENDS_LIST}
+  SWIFT_MODULE_DEPENDS_OSX Darwin
   PRIVATE_LINK_LIBRARIES ${SOURCEKITD_LINK_LIBS}
   SWIFT_COMPILE_FLAGS ${EXTRA_COMPILE_FLAGS}
   INSTALL_IN_COMPONENT ${INSTALLED_COMP}