devel/erlang-exmpp: Use gsed during the build

Parts of erlang-exmpp's build use base sed with expressions that
don't actually work correctly in our sed as of yet. In this case,
the replacement of \xa9 with a © entity. The former should
be interpreted as the character \xa9 (copyright symbol), but sed
does not currently interpret it as such (or \r, \n, \t). Switch
to textproc/gsed for now, which does the right thing.

PR: 233424
Submitted by: kevans
Approved by: erlang (maintainer timeout, 3 months)