um: Fix out-of-tree build

Commit 30b11ee9 (um: Remove copy&paste code from init.h)
uncovered an issue wrt. out-of-tree builds.
For out-of-tree builds, we must not rely on relative paths.
Before 30b11ee9 it worked by chance as no host code included
generated header files.
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/Makefile

@@ -70,8 +70,8 @@ KBUILD_AFLAGS += $(ARCH_INCLUDE)
 
 USER_CFLAGS = $(patsubst $(KERNEL_DEFINES),,$(patsubst -I%,,$(KBUILD_CFLAGS))) \
 		$(ARCH_INCLUDE) $(MODE_INCLUDE) $(filter -I%,$(CFLAGS)) \
-		-D_FILE_OFFSET_BITS=64 -idirafter include \
-		-D__KERNEL__ -D__UM_HOST__
+		-D_FILE_OFFSET_BITS=64 -idirafter $(srctree)/include \
+		-idirafter $(obj)/include -D__KERNEL__ -D__UM_HOST__
 
 #This will adjust *FLAGS accordingly to the platform.
 include $(ARCH_DIR)/Makefile-os-$(OS)