Fix lwip compilation with arm-2010.09-51- toolchain

The error ESHUTDOWN was not recognized with the latest
arm toolchain because it requires a particular define.

This patch defines the __LINUX_ERRNO_EXTENSIONS__ macro
at the top of the lwip.c file because it's required only
to compile lwip.
If needed, it should be defined only for arm toolchains.

git-svn-id: https://src.develer.com/svnoss/bertos/trunk@4650 38d2e660-2303-0410-9eaa-f027e97ec537