selinux: fix misspellings using codespell tool

A typo is found out by codespell tool in 16th line of hashtab.c

$ codespell ./security/selinux/ss/
./hashtab.c:16: rouding  ==> rounding

Fix a typo found by codespell.

Signed-off-by: Xiong Zhenwu <xiong.zhenwu@zte.com.cn>
[PM: subject line tweak]
Signed-off-by: Paul Moore <paul@paul-moore.com>