gnulib: switch to use https:// instead of git:// protocol

Some contributors are behind obnoxious firewalls that block everything
except http(s) traffic, preventing checkout of modules using the git://
protocol. Since git.savannah.gnu.org is using the modern, fast HTTP
transport, there's no real downside to using that by default.
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

.gitmodules

 [submodule "gnulib"]
 	path = .gnulib
-	url = git://git.sv.gnu.org/gnulib.git
+	url = https://git.savannah.gnu.org/git/gnulib.git/
 [submodule "keycodemapdb"]
 	path = src/keycodemapdb
 	url = https://gitlab.com/keycodemap/keycodemapdb.git