lsns: fix const qualifier warnings for C23

lsns: fix const qualifier warnings for C23

Fix const qualifier discarded warnings in read_persistent_namespaces()
and is_path_included() functions. These warnings are reported by gcc 15
which defaults to the C23 standard.

The strchr() and strstr() functions return pointers into const strings,
so the receiving variables must be declared as const char *.

Signed-off-by: Karel Zak <kzak@redhat.com>