kbuild: make samples depend on headers_install

Olof's build test setup keeps failing to compile arm64 kernels
because of a toolchain that uses outdated kernel headers:

/work/build/batch/samples/seccomp/bpf-fancy.c:13:27: fatal error: linux/seccomp.h: No such file or directory

This is of course something he could change, but it also indicates
that others may run into the same problem. Running 'make headers_install'
avoids the issue by ensuring that the kernel headers are put into
the $(objdir)/usr/include path before we build the samples.

The same problem happened for the Documentation build in the
past and was fixed up with commit 8e2faea8 ("Make Documenation
depend on headers_install"). This adds an identical Makefile dependency
for the samples/ subdirectory.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Michal Marek <mmarek@suse.com>