Add "multiset" to keywords for syntax highlighting

With the addition of the multiset builtin message theory to Tamarin,
the vim syntax highlighting should highlight it in the same way as
the other builtin theories.

etc/spthy.vim

@@ -39,7 +39,7 @@ syn match spthyLAtom	        "@"
 syn match spthyLAtom	        "<"
 syn match spthyLAtom	        ">"
 
-syn keyword spthyConstr         aenc sdec senc sdec sign verify hashing signing
+syn keyword spthyConstr         aenc sdec senc sdec sign verify hashing signing multiset
 syn match spthyConstr           "\<h("he=e-1
 syn match spthyConstr           "\<sk("he=e-1
 syn match spthyConstr           "\<pk("he=e-1