um: make sure to have generated headers for target

Signed-off-by: NAl Viro <viro@zeniv.linux.org.uk>

arch/um/Makefile

@@ -116,8 +116,15 @@ endef
 KBUILD_KCONFIG := $(HOST_DIR)/um/Kconfig
 
 archheaders:
+	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.asm-generic \
+	            kbuild-file=$(HOST_DIR)/include/asm/Kbuild \
+		    obj=$(HOST_DIR)/include/generated/asm
+	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.asm-generic \
+	            kbuild-file=$(HOST_DIR)/include/uapi/asm/Kbuild \
+		    obj=$(HOST_DIR)/include/generated/uapi/asm
 	$(Q)$(MAKE) KBUILD_SRC= ARCH=$(HEADER_ARCH) archheaders
 
+
 archprepare: include/generated/user_constants.h
 
 LINK-$(CONFIG_LD_SCRIPT_STATIC) += -static