Fixes broken build due to misplaced import

The GRPCCall import must be in the header because it defines a protocol
that GRPCRequestHeaders uses.

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

@@ -34,7 +34,7 @@
 #import <Foundation/Foundation.h>
 #include <grpc/grpc.h>
 
-@class GRPCCall.h;
+#import "../GRPCCall.h"
 
 @interface GRPCRequestHeaders : NSObject<GRPCRequestHeaders>
 

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

@@ -35,7 +35,6 @@
 
 #import <Foundation/Foundation.h>
 
-#import "../GRPCCall.h"
 #import "NSDictionary+GRPC.h"
 
 // Used by the setter.