util/disk_cache: Fix disk_cache_get_function_timestamp with disabled cache.

radv always needs it, so just check the header instead. Also
do not declare the function if the variable is not set, so we
get a nice compile error instead of failing to open a device
at runtime.

Fixes: b87ef9e6 "util: fix MSVC build issue in disk_cache.h"
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
(cherry picked from commit cc10b34e)