native: -DNATIVE_INCLUDES if using NATIVEINCLUDES

It can be useful to know if the current compilation builds the native
board module, because e.g. then we must not include `sys/types.h`.

This diff adds `-DNATIVE_INCLUDES` to the make variable
`NATIVEINCLUDES`.

boards/native/Makefile.include

@@ -1,6 +1,7 @@
 include $(RIOTBOARD)/$(BOARD)/Makefile.dep
 
-export NATIVEINCLUDES = -I$(RIOTBOARD)/$(BOARD)/include/
+export NATIVEINCLUDES += -DNATIVE_INCLUDES
+export NATIVEINCLUDES += -I$(RIOTBOARD)/$(BOARD)/include/
 export NATIVEINCLUDES += -I$(RIOTBASE)/core/include/
 export NATIVEINCLUDES += -I$(RIOTBASE)/drivers/include/