Corrected cross-platform stat nanosecond code

When nanosecond time resolution was added for identifying config file
changes, it didn't cover all of the myriad of ways that one might obtain
nanosecond time resolution off of struct stat.

Rather than complicate the #if even further figuring out one system from
the next, this patch directly tests for the three struct members I know
about today, and #ifdef's accordingly.

Review: https://reviewboard.asterisk.org/r/3273/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@409833 f38db490-d61c-443f-a65b-d21fe96a405b