samples: timers: use 'userprogs' syntax

Kbuild now supports the 'userprogs' syntax to compile userspace
programs for the same architecture as the kernel.

Add the entry to samples/Makefile to put this into the build bot
coverage.

I also added the CONFIG option guarded by 'depends on CC_CAN_LINK'
because $(CC) may not provide libc.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
diff --git a/samples/Kconfig b/samples/Kconfig
index 2322e11..a8629a0 100644
--- a/samples/Kconfig
+++ b/samples/Kconfig
@@ -135,6 +135,10 @@
 	  Build samples of seccomp filters using various methods of
 	  BPF filter construction.
 
+config SAMPLE_TIMER
+	bool "Timer sample"
+	depends on CC_CAN_LINK && HEADERS_INSTALL
+
 config SAMPLE_UHID
 	bool "UHID sample"
 	depends on CC_CAN_LINK && HEADERS_INSTALL