Fix future version of the Apple toolchain failure (#25611 An error occurred )

More detail at https://github.com/firebase/firebase-ios-sdk/pull/7563

src/core/tsi/alts/crypt/gsec.h

@@ -29,12 +29,14 @@
 
 #include <grpc/grpc.h>
 
+#ifndef _STRUCT_IOVEC
 #ifndef GRPC_EVENT_ENGINE_POSIX
 struct iovec {
   void* iov_base;
   size_t iov_len;
 };
 #endif  // GRPC_EVENT_ENGINE_POSIX
+#endif  // _STRUCT_IOVEC
 
 /**
  * A gsec interface for AEAD encryption schemes. The API is thread-compatible.