Resolved warning "call_in inherits implicit virtual"

Author: deepikabhavnani

features/lwipstack/LWIPStack.h

@@ -450,7 +450,7 @@ class LWIP : public OnboardNetworkStack, private mbed::NonCopyable<LWIP> {
      *  @param func     Callback to be called
      *  @return         0 on success, negative error code on failure
      */
-    nsapi_error_t call_in(int delay, mbed::Callback<void()> func);
+    virtual nsapi_error_t call_in(int delay, mbed::Callback<void()> func);
 
     struct mbed_lwip_socket {
         bool in_use;