README: Add cmake dependency

cmake is an optional dependency for vtpm.

Signed-off-by: Matthew Fioravante <matthew.fioravante@jhuapl.edu>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>

README

@@ -65,6 +65,7 @@ disabled at compile time:
     * Development install of Ocaml (e.g. ocaml-nox and
       ocaml-findlib). Required to build ocaml components which
       includes the alternative ocaml xenstored.
+    * cmake (if building vtpm stub domains)
 
 Second, you need to acquire a suitable kernel for use in domain 0. If
 possible you should use a kernel provided by your OS distributor. If