nanostack: silence compiler warnings for implicit virtual call_in

ARMCC reminds of the missing virtual declaration of call_in in the
Nanostack.h.

Warnings silenced:
---8<---8<---
Compile [ 52.3%]: LoWPANNDInterface.cpp
[Warning] Nanostack.h@278,0:  #1300-D: call_in inherits implicit virtual
Compile [ 52.4%]: NanostackEMACInterface.cpp
[Warning] Nanostack.h@278,0:  #1300-D: call_in inherits implicit virtual
Compile [ 52.5%]: MeshInterfaceNanostack.cpp
[Warning] Nanostack.h@278,0:  #1300-D: call_in inherits implicit virtual
Compile [ 52.6%]: NanostackEthernetInterface.cpp
[Warning] Nanostack.h@278,0:  #1300-D: call_in inherits implicit virtual
Compile [ 52.7%]: ThreadInterface.cpp
[Warning] Nanostack.h@278,0:  #1300-D: call_in inherits implicit virtual
ompile [ 53.0%]: WisunInterface.cpp
[Warning] Nanostack.h@278,0:  #1300-D: call_in inherits implicit virtual

Author: TeroJaasko

features/nanostack/nanostack-interface/Nanostack.h

@@ -275,7 +275,7 @@ class Nanostack : public OnboardNetworkStack, private mbed::NonCopyable<Nanostac
      *  @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 nanostack_callback {
         mbed::Callback<void()> callback;