Commit 54b733ce9a0d1cd4ab9b142479368a5e696360be broke compilation of
echld by removing TS_PREC_AUTO_USEC. Replace its use by TS_PREC_AUTO.
No idea whether that's correct but it compiles again.
Trying to commit this brought complaints by checkapi, so fixed them as
well.

Maybe the echld code should be removed, it seems to be unmaintained.

Change-Id: Id9a0a6fc129abd1b8c93629c31a89c2d9ea2243f
Reviewed-on: https://code.wireshark.org/review/4811
Reviewed-by: Jörg Mayer <jmayer@loplof.de>