[2726] Suppress false positive cppcheck warning

src/lib/dhcp/option_custom.h

@@ -309,6 +309,8 @@ private:
     /// @tparam data type to be validated.
     ///
     /// @throw isc::dhcp::InvalidDataType if the type is invalid.
+    //cppcheck-suppress unusedPrivateFunction It's used, confusion because of
+    //templates
     template<typename T>
     void checkDataType(const uint32_t index) const;