diff: Show nanoseconds in -u/-c header line.

Show nanoseconds in the -u/-c header line.

The present portability conditionals cannot handle the POSIX standard
st_mtim, so remove them and unconditionally use st_mtim.

PR: 218018
Reported by: jbeich
Reviewed by: bapt
Differential Revision: https://reviews.freebsd.org/D10145