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.
diff --git a/objectivec/GPBMessage_PackagePrivate.h b/objectivec/GPBMessage_PackagePrivate.h
index c5501c8..820bb7b 100644
--- a/objectivec/GPBMessage_PackagePrivate.h
+++ b/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 {
