selinux: declare read-only data arrays const

The array of mount tokens in only used in match_opt_prefix() and never
modified.

The array of symtab names is never modified and only used in the
DEBUG_HASHES configuration as output.

The array of files for the SElinux filesystem sub-directory `ss` is
similar to the other `struct tree_descr` usages only read from to
construct the containing entries.

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>