[libdispatch] Linux build fixes

    * Dispatch's shims header file was not adequately protected against
    building on Linux (which does not have the blocks runtime).

Author: mwwa

stdlib/public/SwiftShims/DispatchShims.h

@@ -19,7 +19,7 @@
 #ifndef SWIFT_STDLIB_SHIMS_DISPATCHSHIMS_H
 #define SWIFT_STDLIB_SHIMS_DISPATCHSHIMS_H
 
-//#include <dispatch/dispatch.h>
+#ifdef __OBJC2__
 
 #include "SwiftStdint.h"
 #include "SwiftStddef.h"
@@ -99,5 +99,7 @@ void _swift_dispatch_apply_current(
 }} // extern "C", namespace swift
 #endif
 
+#endif // __OBJC2__
+
 #endif // SWIFT_STDLIB_SHIMS_DISPATCHSHIMS_H