Replace '#ifdef CONFIG_...' with '#if CONFIG_...'

This affects two places:
* Fixes a compile error with frame-superres when
  highbitdepth is disabled.
* Avoids including some supertx-related code when supertx
  is disabled

BUG=aomedia:602

Change-Id: Idfc478fd88ade91d48c93cfd8abdd2bea86de898