[ObjC] Use a real import for GPBExtensionRegistry.
The fwd decls can lead to ambiguity for downstream users, so use a full import
to avoid the potential issues.
diff --git a/objectivec/GPBCodedInputStream.h b/objectivec/GPBCodedInputStream.h
index 2b95cc9..b7e1c7e 100644
--- a/objectivec/GPBCodedInputStream.h
+++ b/objectivec/GPBCodedInputStream.h
@@ -30,9 +30,9 @@
#import <Foundation/Foundation.h>
+#import "GPBExtensionRegistry.h"
+
@class GPBMessage;
-@class GPBExtensionRegistry;
-@protocol GPBExtensionRegistry;
NS_ASSUME_NONNULL_BEGIN