selftests/ipc: Fix msgque compiler warnings

selftests/ipc: Fix msgque compiler warnings

[ Upstream commit a147faa96f832f76e772b1e448e94ea84c774081 ]

This fixes the various compiler warnings when building the msgque
selftest. The primary change is using sys/msg.h instead of linux/msg.h
directly to gain the API declarations.

Fixes: 3a665531a3b7 ("selftests: IPC message queue copy feature test")
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>