samples/seccomp: Add standalone config option

Add a separate Kconfig option for SAMPLES_SECCOMP.

Main reason for this is that, just like other samples, it's forced to
be a module.

Without this, since the sample is a target only controlled by
CONFIG_SECCOMP_FILTER, the samples will be built before include files are
put in place properly. For example, from an arm64 allmodconfig built with
"make -sk -j 32" (without specific target), the following happens:

  samples/seccomp/bpf-fancy.c:13:27: fatal error: linux/seccomp.h: No such file or directory
  samples/seccomp/bpf-helper.h:20:50: fatal error: linux/seccomp.h: No such file or directory
  samples/seccomp/dropper.c:20:27: fatal error: linux/seccomp.h: No such file or directory
  samples/seccomp/bpf-direct.c:21:27: fatal error: linux/seccomp.h: No such file or directory

So, just stick to the same format as other samples.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Kees Cook <keescook@chromium.org>