random: note that RNDGETPOOL was removed in 2.6.9-rc2

RNDGETPOOL was thankfully removed twenty years ago, but it's stuck
around in headers. Probably removing it from uapi headers isn't great in
case there are some weird users out there, but we should at least mark
this as having been removed, to save future readers the same goose chase
I just went on.

Link: https://lore.kernel.org/all/E1By1St-0001TS-Qj@thunk.org/
Link: https://lore.kernel.org/all/Pine.LNX.4.58.0409130937050.4094@ppc970.osdl.org/


Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>