samples/seccomp: Enable PR_SET_NO_NEW_PRIVS in dropper

Either CAP_SYS_ADMIN or PR_SET_NO_NEW_PRIVS is required to enable
seccomp. This allows samples/seccomp/dropper to be run without
CAP_SYS_ADMIN.

Signed-off-by: Ricky Zhou <rickyz@chromium.org>
Signed-off-by: Kees Cook <keescook@chromium.org>