Don't use racket/private/namespace (easy)

Summary:

Instead of requiring `racket/private/namespace` for make-base-namespace,
just grab it with `(only racket/base make-base-namespace)` instead.

Test Plan:

With the file open in DrRacket, a single `make-base-namespace` call
was the only shown bound occurrence of the require.

Author: jesboat

drracket/gui-debugger/load-sandbox.rkt

@@ -2,7 +2,7 @@
 
   (require syntax/moddep
            mzlib/class
-           racket/private/namespace
+           (only racket/base make-base-namespace)
            mred)  
 
   (provide eval/annotations