Do not declare `headers` an alias to `libs/headers/build/boost_headers`

Jamroot

@@ -244,7 +244,7 @@ explicit-alias serialization : libs/serialization/build//boost_serialization ;
 explicit-alias wserialization : libs/serialization/build//boost_wserialization ;
 for local l in $(all-libraries)
 {
-    if ! $(l) in test graph serialization
+    if ! $(l) in test graph serialization headers
     {
         explicit-alias $(l) : libs/$(l)/build//boost_$(l) ;
     }