Fix Linux build.

Greg Parker · Greg Parker · commit 5ed05f55c5ce · 2016-12-14T18:35:05.000-08:00
diff --git a/stdlib/public/SwiftShims/RefCount.h b/stdlib/public/SwiftShims/RefCount.h
@@ -29,6 +29,7 @@ typedef struct {
 
 #include <type_traits>
 #include <atomic>
+#include <stddef.h>
 #include <stdint.h>
 #include <assert.h>
 
