rf_uhd_generic: add explicit ctor and dtor

clang 6 complained about a missing virtual dtor and thus
failed compilation. this patch adds both, an explicit ctor and dtor.

lib/src/phy/rf/rf_uhd_generic.h

@@ -111,6 +111,8 @@ private:
   }
 
 public:
+  rf_uhd_generic(){};
+  virtual ~rf_uhd_generic(){};
   uhd_error usrp_make(const uhd::device_addr_t& dev_addr_, uint32_t nof_channels) override
   {
     uhd::device_addr_t dev_addr = dev_addr_;