gn2bp: fix _rebase_directory

So it actually does anything now. :)

Test: none
Change-Id: Ifa5169e7458b1b1ae4a068215072e9b83a74e117

tools/gn2bp/gen_android_bp

@@ -676,7 +676,7 @@ class BaseActionSanitizer():
 
   # deletes the leading ../../
   def _rebase_directory(self, filename):
-    filename = re.sub('^\.\./\.\./', '', filename)
+    return re.sub('^\.\./\.\./', '', filename)
 
   def get_args(self):
     return self.target.args