background: revert false indentation

Commit 46673670 ("Fix indentation") indented lines 160-163 of cc-background-chooser.c to make them seem as if they were part of the associated if statement. However, they are not, which causes confusion as to the intention of the code. It appears as if the gtk_accessible_update_property call is executed upon button being non- null, but it is always executed due to the lack of an if block. For clarity, and to resolve a compiler warning, this patch should be reverted.

This patch reverts 46673670, changing the indentation back correctly. This was mentioned in #1902 (closed) and received no objections, including approval from Lukas.