compat_freebsd4: Add const qualifier to the local variable s inside function freebsd4_uname()

This local variable s is for iterating characters of global variable
version. The content of version is not going to be altered by
function freebsd4_uname().

MFC after: 1 week

(cherry picked from commit 92f2a4c820fea9efcb6ee6a7029495c8152958c1)