package/opentyrian: fix build on bfin

There is a conflict with the _strchrnul symbole from mingw_fixes.c
and the one from uClibc.

Add a guard around strchrnul function for unix systems.

Fixes:
http://autobuild.buildroot.net/results/a08/a085fb55269971e3c7b8ae8c167e7330c3c042a5/



Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>