Avoid the use of the non-portable -D argument to ls.

This was used to store the mtime of the source file in a commment in a
generated header file. This is of little-to-no diagnostic value and
the result doesn't even end up in the source tree.

Reported by: arichardson
Reviewed by: arichardson
MFC after: 1 days
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D21605