gnuradio: Add libsndfile for gr-blocks

pkgs/applications/radio/gnuradio/default.nix

···
     };
     gr-blocks = {
       cmakeEnableFlag = "GR_BLOCKS";
+      runtime = [
+        # Required to compile wavfile blocks.
+        libsndfile
+      ];
     };
     gr-fec = {
       cmakeEnableFlag = "GR_FEC";