net/qt5-network: fix build with LibreSSL 3.3.2

Since LibreSSL's libssl now exposes DTLS1_2_VERSION, configure
detects DTLS support which changes the DTLSv1_listen(3) signature
to one that LibreSSL has never supported. Force disable DTLS feature
for SSL_DEFAULT=libressl or libressl-devel.

Based on: https://github.com/openbsd/ports/commit/9942e8d28c4ec59d8d4a20b8bf219b10573c1f31

Reviewed by: tcberner (kde, mentor)
Approved by: fluffy (kde, mentor)
Differential Revision: https://reviews.freebsd.org/D29877