Bring back import of OSAtomic.

Followup to https://github.com/google/protobuf/pull/4184, keep the
import to not break any existing generated code that isn't regenerated
when they update to the newer protobuf code.

objectivec/GPBMessage_PackagePrivate.h

@@ -36,6 +36,12 @@
 
 #import <stdatomic.h>
 
+// TODO: Remove this import. Older generated code use the OSAtomic* apis,
+// so anyone that hasn't regenerated says building by having this. After
+// enough time has passed, this likely can be removed as folks should have
+// regenerated.
+#import <libkern/OSAtomic.h>
+
 #import "GPBBootstrap.h"
 
 typedef struct GPBMessage_Storage {