meson: Correctly validate that prefix is a child of rootprefix

In commit d895e10a a test was introduced to validate that prefix is a
child of rootprefix. However, it only works when rootprefix is "/".
Since the test is ignored when rootprefix is equal to prefix, this is
only noticed if specifying both -Drootprefix= and -Dprefix=, e.g.:

  $ meson foo -Drootprefix=/foo -Dprefix=/foo/bar
  meson.build:111:8: ERROR: Problem encountered: Prefix is not below
  root prefix (now rootprefix=/foo prefix=/foo/bar)