msgctl.2, msgget.2, msgop.2, semctl.2, semget.2, semop.2, shmctl.2, shmget.2: NOTES...

msgctl.2, msgget.2, msgop.2, semctl.2, semget.2, semop.2, shmctl.2, shmget.2: NOTES: <sys/types.h> and <sys/ipc.h> aren't strictly needed

Add text to NOTES to say that the <sys/types.h> and <sys/ipc.h>
header files aren't required by Linux or the standards, but may
be needed for portability to old systems.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>