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>

cpu/ixp/npe/Makefile

@@ -25,7 +25,7 @@ include $(TOPDIR)/config.mk
 
 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)