PM /devfreq: Fix function prototype

Replace #ifdef with IS_ENABLED to make sure that the memlat prototypes
are available for a module build.

Fixes: a4e0d26f ("PM / devfreq: Introduce a memory-latency governor")
Change-Id: Ic0dedbade40e23bb148bbb9117c4a9b63e419111
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>