net/kitinerary: prepare for poppler 21.08.0

Add upstream fix [1]:
parseDateString() now takes "const GooString *" as the date
argument in order to support unicode date strings. Adjust
parsePdfDateTime() to use the same string type (instead of
"const char *") when compiling against poppler 21.8.0.

[1] https://invent.kde.org/pim/kitinerary/-/commit/e99d101e2bd4c32566e238ce1340a93781e19eee

PR: 257539