soapybladerf: fix license

pkgs/applications/radio/soapybladerf/default.nix

···
   meta = with lib; {
     homepage = "https://github.com/pothosware/SoapyBladeRF";
     description = "SoapySDR plugin for BladeRF devices";
-    license = licenses.lgpl21;
+    license = licenses.lgpl21Only;
     maintainers = with maintainers; [ markuskowa ];
     platforms = platforms.linux;
   };