[LinuxCNC/linuxcnc PR#497] Fix RTAI 4 support and add rtai_shm

I have no idea how LinuxCNC worked before
without keeping track of rtai_shm.ko as
this is required by rtapi.

rtshmfree and rtshmalloc are required symbols
but these were previously missing.

Signed-off-by: Alec Ari