PM / devfreq: Fix CONFIG check for cache HW monitor prototype

Commit 2094bed7 ("PM / devfreq: Add cache HW monitor governor") brought
in the cache HW monitor with an incorrect check for
CONFIG_DEVFREQ_GOV_QCOM_CACHE_HWMON that fails when the governor is
built as a module.

Change-Id: Ic0dedbadefc4885486660d32226e6dd89e006a11
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>