libblkid: fix const qualifier warning in blkid_parse_tag_string

libblkid: fix const qualifier warning in blkid_parse_tag_string

Fix const qualifier discarded warning in blkid_parse_tag_string().
This warning is reported by gcc 15 which defaults to the C23 standard.

The strchr() function returns a pointer into a const string, so
introduce a separate 'eq' variable to hold this const pointer for
finding the '=' separator. Also move the 'cp' variable declaration
into the block where it's actually used for quote handling.

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