scons: Use Dir().Dir() and not os.path to extend CPPPATH.

Since we're already working with Dir nodes, they can figure out
appending to a path themselves without using os.path.join.

Change-Id: Ib46946d7ec181dbbf443f957f23196eb0fd7f6b5
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/48127
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

src/SConscript

@@ -694,8 +694,7 @@ for extra_dir in extras_dir_list:
 
     # Also add the corresponding build directory to pick up generated
     # include files.
-    env.Append(CPPPATH=Dir(os.path.join(env['BUILDDIR'],
-                    extra_dir[prefix_len:])))
+    env.Append(CPPPATH=Dir(env['BUILDDIR']).Dir(extra_dir[prefix_len:]))
 
     for root, dirs, files in os.walk(extra_dir, topdown=True):
         # if build lives in the extras directory, don't walk down it