www/libnghttp2: Update to 1.55.1

• Switch back to CMake for consistency with www/nghttp2
• Refresh CMake args

Changelog: https://github.com/nghttp2/nghttp2/releases/tag/v1.55.1

PR: 272214
Approved by: portmgr (maintainer timeout, 2+ weeks)
Sponsored by: Blinkinblox