kbuild: make samples really depend on headers_install

Kernel headers must be installed into $(objtree)/usr/include to avoid
the build failure of samples.

Commit ddea05fa ("kbuild: make samples depend on headers_install")
addressed this, but "samples/" is only used for the single target build.

"make samples/" properly installs kernel headers, but it does not work
for general building because a phony target "sample" (no trailing slash)
is used.

Reported-by: David Howells <dhowells@redhat.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: David Howells <dhowells@redhat.com>