Ensure all overridden virtual functions are marked with override

This also removes warnings.

Author: hugueskamba

features/lwipstack/LWIPStack.h

@@ -341,7 +341,7 @@ class LWIP : public OnboardNetworkStack, private mbed::NonCopyable<LWIP> {
     nsapi_error_t add_dns_server(const SocketAddress &address, const char *interface_name) override;
 
     /** @copydoc NetworkStack::get_ip_address */
-    virtual nsapi_error_t get_ip_address(SocketAddress *address);
+    nsapi_error_t get_ip_address(SocketAddress *address) override;
 
     /** @copydoc NetworkStack::get_ip_address_if */
     nsapi_error_t get_ip_address_if(SocketAddress *address, const char *interface_name) override;

features/nanostack/nanostack-interface/Nanostack.h

@@ -50,7 +50,7 @@ class Nanostack : public OnboardNetworkStack, private mbed::NonCopyable<Nanostac
     /* Local variant with stronger typing and manual address specification */
     nsapi_error_t add_ppp_interface(PPP &ppp, bool default_if, Nanostack::PPPInterface **interface_out);
 
-    nsapi_error_t remove_ppp_interface(OnboardNetworkStack::Interface **interface_out);
+    nsapi_error_t remove_ppp_interface(OnboardNetworkStack::Interface **interface_out) override;
 
 protected:
 

features/netsocket/ppp/ppp_service.h

@@ -138,7 +138,7 @@ class ppp_service final : public PPP {
      * @param uname    User name
      * @param password Password
      */
-    void set_credentials(const char *uname, const char *password);
+    void set_credentials(const char *uname, const char *password) override;
 
     /** Gets local IP address
      *