ixp: add missing os define

need by arm-elf toolchains and no impact on the arm-linux one

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
diff --git a/cpu/ixp/npe/Makefile b/cpu/ixp/npe/Makefile
index 9309f37..f4f97bd 100644
--- a/cpu/ixp/npe/Makefile
+++ b/cpu/ixp/npe/Makefile
@@ -25,7 +25,7 @@
 
 LIB := $(obj)libnpe.a
 
-LOCAL_CFLAGS  += -I$(TOPDIR)/cpu/ixp/npe/include -DCONFIG_IXP425_COMPONENT_ETHDB
+LOCAL_CFLAGS  += -I$(TOPDIR)/cpu/ixp/npe/include -DCONFIG_IXP425_COMPONENT_ETHDB -D__linux
 CFLAGS  += $(LOCAL_CFLAGS)
 HOST_CFLAGS  += $(LOCAL_CFLAGS)