samples: watch_queue: build sample program for target architecture

This userspace program includes UAPI headers exported to usr/include/.
'make headers' always works for the target architecture (i.e. the same
architecture as the kernel), so the sample program should be built for
the target as well. Kbuild now supports 'userprogs' for that.

I also guarded the CONFIG option by 'depends on CC_CAN_LINK' because
$(CC) may not provide libc.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

samples/Kconfig

@@ -211,7 +211,7 @@ config SAMPLE_WATCHDOG
 
 config SAMPLE_WATCH_QUEUE
 	bool "Build example /dev/watch_queue notification consumer"
-	depends on HEADERS_INSTALL
+	depends on CC_CAN_LINK && HEADERS_INSTALL
 	help
 	  Build example userspace program to use the new mount_notify(),
 	  sb_notify() syscalls and the KEYCTL_WATCH_KEY keyctl() function.

samples/watch_queue/Makefile

-# List of programs to build
-hostprogs := watch_test
+# SPDX-License-Identifier: GPL-2.0-only
+userprogs := watch_test
+always-y := $(userprogs)
 
-# Tell kbuild to always build the programs
-always-y := $(hostprogs)
-
-HOSTCFLAGS_watch_test.o += -I$(objtree)/usr/include
+userccflags += -I usr/include