Add Accept http header

Fix broken connection to netfilter.org which refuse connection without the header.

DEBUG: loading config file at: /home/seblu/.config/archversion/packages.conf
DEBUG: Loading database /home/seblu/.cache/archversion/packages.cache
DEBUG: Syncing versions of package libnftnl
DEBUG: Get upstream version
DEBUG: Requesting url: https://www.netfilter.org/projects/libnftnl/files/ (try 1/4)
DEBUG: Timeout is 30.0
DEBUG: Requesting url: https://www.netfilter.org/projects/libnftnl/files/ (try 2/4)
DEBUG: Timeout is 30.0
DEBUG: Requesting url: https://www.netfilter.org/projects/libnftnl/files/ (try 3/4)
DEBUG: Timeout is 30.0
DEBUG: Requesting url: https://www.netfilter.org/projects/libnftnl/files/ (try 4/4)
DEBUG: Timeout is 30.0
ERROR: Sync of libnftnl: Upstream check failed: HTTP Error 403: Forbidden
DEBUG: Comparing versions of package libnftnl