linux: fix ntp_gettime abi break (BZ# 30156)

Between versions v2.11 and v2.12 struct ntptimeval got new fields.
That wasn't a problem because new function ntp_gettimex was created
(and made default) to support new struct.  Old ntp_gettime was not
using new fields so it was safe to call with old struct
definition.  Then commits 5613afe9 and b6ad64b9 (added for
64 bit time_t support), ntp_gettime start setting new fields.

Sets fields manually to maintain compatibility with v2.11 struct
definition.

Resolves #30156

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>