whereis: fix const qualifier warnings for C23

whereis: fix const qualifier warnings for C23

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

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

Signed-off-by: Karel Zak <kzak@redhat.com>
(cherry picked from commit 01018b74163f9122c75179a7c991b0aa0f8c603c)