[nrf toup][nrfconnect] Allow inheriting DiagnosticDataProviderImplNrf

The default constructor of DiagnosticDataProviderImplNrf must be
protected instead of private to allow inheriting it in the
user's implementation.

Signed-off-by: Arkadiusz Balys <[email protected]>

Author: ArekBalysNordic

src/platform/nrfconnect/DiagnosticDataProviderImplNrf.h

@@ -50,7 +50,7 @@ class DiagnosticDataProviderImplNrf : public DiagnosticDataProviderImpl
 
     static DiagnosticDataProviderImplNrf & GetDefaultInstance();
 
-private:
+protected:
     DiagnosticDataProviderImplNrf() = default;
 };