gnu: hitch: Provide the 'ps' command for tests.

...otherwise, failing tests would show up as successful.  This is a follow-up
to commit 273380fda2.

* gnu/packages/web.scm (hitch)[native-inputs]: Add PROCPS.

gnu/packages/web.scm

@@ -5895,6 +5895,7 @@ on the fly.")
        ("curl" ,curl)
        ("egrep" ,grep)
        ("lsof" ,lsof)
+       ("procps" ,procps)
        ("python" ,python)))
     (inputs
      `(("libev" ,libev)