samples: disable CONFIG_SAMPLES for UML

Some samples require headers installation, so commit 3fca1700c4c3
("kbuild: make samples really depend on headers_install") added
such dependency in the top Makefile. However, UML fails to build
with CONFIG_SAMPLES=y because UML does not support headers_install.

Fixes: 3fca1700c4c3 ("kbuild: make samples really depend on headers_install")
Reported-by: Kees Cook <keescook@chromium.org>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/samples/Kconfig b/samples/Kconfig
index bd133ef..ad1ec701 100644
--- a/samples/Kconfig
+++ b/samples/Kconfig
@@ -1,5 +1,6 @@
 menuconfig SAMPLES
 	bool "Sample kernel code"
+	depends on !UML
 	help
 	  You can build and test sample kernel code here.