i2pd: fix meta.license

pkgs/tools/networking/i2pd/default.nix

···
   meta = with stdenv.lib; {
     homepage = https://i2pd.website;
     description = "Minimal I2P router written in C++";
-    license = licenses.gpl2;
+    license = licenses.bsd3;
     maintainers = with maintainers; [ edwtjo ];
     platforms = platforms.linux;
   };