Fixes ocaml 4 compatibility.

From upstream:
svn diff http://ocaml-extlib.googlecode.com/svn/trunk/ -c 396

Index: extlib/extHashtbl.ml
===================================================================
--- extlib/extHashtbl.ml	(revision 395)
+++ extlib/extHashtbl.ml	(revision 396)
@@ -32,6 +32,7 @@
 	}
 
 	include Hashtbl
+	let create n = Hashtbl.create (* no seed *) n
 
 	external h_conv : ('a, 'b) t -> ('a, 'b) h_t = "%identity"
 	external h_make : ('a, 'b) h_t -> ('a, 'b) t = "%identity"