tools/primreq: Fix build bustage due to string vs bytes (Demi Obenour)

tools/primreq.ml

@@ -40,8 +40,7 @@ let scan_info cu =
 
 let scan_obj filename =
   let ic = open_in_bin filename in
-  let buffer = String.create (String.length cmo_magic_number) in
-  really_input ic buffer 0 (String.length cmo_magic_number);
+  let buffer = really_input_string ic (String.length cmo_magic_number) in
   if buffer = cmo_magic_number then begin
     let cu_pos = input_binary_int ic in
     seek_in ic cu_pos;