libvirt: fix dlopen("libjansson.so.4")

Commit 59027e2 broke libvirt broke libvirt/nixops on linux.
Use ${targetPlatform.extensions.sharedLibrary} to accomodate both linux and darwin.

pkgs/development/libraries/libvirt/default.nix

@@ -60,7 +60,9 @@ in stdenv.mkDerivation rec {
       --replace 'lxc_path,' '"/run/libvirt/nix-emulators/libvirt_lxc",'
 
     patchShebangs . # fixes /usr/bin/python references
-  '';
+    substituteInPlace src/util/virjsoncompat.c --replace \
+        '"libjansson.so.4"' '"${jansson}/lib/libjansson${stdenv.targetPlatform.extensions.sharedLibrary}"'
+   '';
 
   configureFlags = [
     "--localstatedir=/var"