pounce: replace libressl with libretls

pounce cannot find the system ca store using latest libressl:

pounce: tls_connect: failed to open CA file 'etc/ssl/cert.pem': No such file or directory

The solution here is the same one taken by the catgirl package, see bcff49197e.

pkgs/by-name/po/pounce/package.nix

@@ -1,7 +1,8 @@
 {
   lib,
   stdenv,
-  libressl,
+  libretls,
+  openssl,
   fetchzip,
   pkg-config,
   libxcrypt,
@@ -17,7 +18,8 @@ stdenv.mkDerivation (finalAttrs: {
   };
 
   buildInputs = [
-    libressl
+    libretls
+    openssl
     libxcrypt
   ];