selftests: openat2: Add missing dependency in Makefile

Add a dependency on header helpers.h to the main target; while at that add
to helpers.h also a missing include for bool types.

Cc: Aleksa Sarai <cyphar@cyphar.com>
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>