samples: disable CONFIG_SAMPLES for UML

Some samples require headers installation, so commit 3fca1700
("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: 3fca1700 ("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>