I would like to report that by running this version of the tool:
PExports 0.46; Originally written 1998, Anders Norlander
Updated 1999, Paul Sokolovsky, 2008, Tor Lillqvist, 2013, Keith Marshall
Copyright (C) 1998, 1999, 2008, 2013, MinGW.org Project
the strnlen() is actually listed with all other functions... perhaps its absence was caused by a bug in this tool but nowadays it seems to be fixed.
EDIT: Keith, you are right. I forgot to mention that I did the test on Windows Seven Home Premium.