libdvdnav: fix nios2 cross-compile

The host is misdetected as OS/2.

Fixes:
http://autobuild.buildroot.net/results/d5b30d6b5463928ded1a1c1f1f5345a726f6ffaf/
http://autobuild.buildroot.net/results/917041c7165093f44a5d3b4a5ba56a3d1b59e456/
http://autobuild.buildroot.net/results/dcfc53b6fc25210cf70a1abf0e24b85f0d971101/
http://autobuild.buildroot.net/results/79109d06183efa17f6f68c1ea78cbd9927c5c46e/



Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Tested-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>