cmd/go: add fossil to general server regexp in get

cmd/go: add fossil to general server regexp in get

Fix a missed change from:

https://golang.org/cl/56190

pointed out on the fossil mailing list shortly after submission
of the change mentioned above.  See:

http://www.mail-archive.com/fossil-users@lists.fossil-scm.org/msg25736.html

This change adds fossil to the general regular expression that is checked last
in the import path check.

For #10010

Change-Id: I6b711cdb1a8d4d767f61e1e28dc29dce529e0fad
Reviewed-on: https://go-review.googlesource.com/56491
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>