selftests/ipc: Fix msgque compiler warnings

commit a147faa9 upstream.

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>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>