Doc: add missing include in usage example

src/unpack.hh

@@ -65,7 +65,7 @@ namespace SignedPackage {
 /*  how to provision for a user and use this API
 
     #include <pEp/unpack.hh>
-
+    #include <cryptopp/xed25519.h>
     void provision(std::string url)
     {
         pEp::UpdateClient::product p { "provisioning data", url };