devtool: fix update-recipe dry-run mode

devtool: fix update-recipe dry-run mode

When running devtool update-recipe with --mode=srcrev AND --append switch
in dry-run, we get the following error:

  Traceback (most recent call last):
  [...]
  Exception: destpath should be set here

Fix this by removing a misplaced else statement in _update_recipe_srcrev

Signed-off-by: Julien Stephan <jstephan@baylibre.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>