x86: fix up system call numbering nit

As pointed out by Jiri Slaby: when I resolved the the 32-bit x85 system
call entry tables for prlimit (due to the conflict with fanotify), I
forgot to add the numbering in comments that we do for every fifth entry.

Reported-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>