selftests/ipc: Fix msgque compiler warnings

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: 3a665531

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