security/gnupg: Add pkg-message about potential connectivity issues

GnuPG, when run on hosts without IPv6 connectivity, may fail to connect to
dual-stack hkp servers [1]. Describe a workaround in pkg-message.

[1] https://dev.gnupg.org/rGecfc4db3a2f8bc2652ba4ac4de5ca1cd13bfcbec

Approved by: adamw (maintainer)
Differential Revision: https://reviews.freebsd.org/D17071