dmesg: fix const qualifier warnings in parse_callerid

dmesg: fix const qualifier warnings in parse_callerid

Fix const qualifier discarded warnings in parse_callerid().
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>