shm: Make shm_pool_interface static const

Add static const modifiers to the shm_pool_interface definition,
making it consistent with the other wl_*_interface definitions and
mundanely safer.

Note that this does not affect the ABI, according to abi-dumper and
abi-compliance-checker[1]; and weston and its shm-backed clients still
run as expected.

[1]: http://lvc.github.io/abi-compliance-checker/



Signed-off-by: Yong Bakos <ybakos@humanoriented.com>
Tested-by: Yong Bakos <ybakos@humanoriented.com>
Acked-by: Bryce Harrington <bryce@osg.samsung.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>