utils: timeutil: option to disable clock skew

Accurately applying clock skew correction in
`timeutil_sync_ref_from_local` and `timeutil_sync_local_from_ref`
requires double-precision floating point operations. If this is the only
usage of double precision logic in the build, but clocks skews are not
used, this can be a significant ROM overhead. Disabling this option can
save ~2.4 kB of ROM.

Signed-off-by: Jordan Yates <jordan@embeint.com>