Undo forward-declaring grpc_channel_args, which isn’t a struct!

It’s a typedef of an anonymous struct.

src/objective-c/GRPCClient/private/GRPCSecureChannel.h

@@ -31,11 +31,12 @@
  *
  */
 
-struct grpc_credentials;
+#include <grpc/grpc.h>
-struct grpc_channel_args;
 
 #import "GRPCChannel.h"
 
+struct grpc_credentials;
+
 @interface GRPCSecureChannel : GRPCChannel
 - (instancetype)initWithHost:(NSString *)host;
 
@@ -47,5 +48,5 @@ struct grpc_channel_args;
 
 - (instancetype)initWithHost:(NSString *)host
                  credentials:(struct grpc_credentials *)credentials
-                        args:(struct grpc_channel_args *)args NS_DESIGNATED_INITIALIZER;
+                        args:(grpc_channel_args *)args NS_DESIGNATED_INITIALIZER;
 @end

src/objective-c/GRPCClient/private/GRPCSecureChannel.m

@@ -33,7 +33,7 @@
 
 #import "GRPCSecureChannel.h"
 
-#import <grpc/grpc_security.h>
+#include <grpc/grpc_security.h>
 
 static grpc_credentials *CertificatesAtPath(NSString *path) {
   NSData *certsData = [NSData dataWithContentsOfFile:path];