logger: fix const qualifier warnings for C23

logger: fix const qualifier warnings for C23

Fix const qualifier discarded warnings in valid_structured_data_param()
and valid_structured_data_id() 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>