.gitmodules: switch to HTTPS

HTTPS is not only encrypted but also better suited for corporate
firewalls. Let's use it to clone submodules.
Copyright-paperwork-exempt: yes

.gitmodules

@@ -1,6 +1,6 @@
 [submodule "gnulib"]
 	path = gnulib
-	url = git://git.sv.gnu.org/gnulib.git
+	url = https://git.savannah.gnu.org/git/gnulib.git
 [submodule "paxutils"]
 	path = paxutils
-	url = git://git.sv.gnu.org/paxutils.git
+	url = https://git.savannah.gnu.org/git/paxutils.git