contrib/jenkins.sh: clone libnftnl and libnftables via git://

From time to time we see sporadic master build failures on Jenkins
because git fails to clone one of the repositories:

```
Cloning into 'nftables'...
error: garbage at end of loose object '0ca03ecd6ab3cfdc94f8f9ef6e3a7c40d1aa7195'
fatal: loose object 0ca03ecd6ab3cfdc94f8f9ef6e3a7c40d1aa7195
(stored in /build/libnftnl/nftables/.git/objects/0c/a03ecd6ab3cfdc94f8f9ef6e3a7c40d1aa7195) is corrupt

Cloning into 'libnftnl'...
fatal: unable to access 'https://git.netfilter.org/libnftnl/':
Failed to connect to git.netfilter.org port 443: Connection timed out
```

Running git with GIT_CURL_VERBOSE=true reveals that the server is using
an old "dumb" git protocol, so the client is sending hundreds of HTTP
requests to the server.  I also noticed that cloning via http[s]://
takes significantly more time than cloning via git://, because of the
old protocol being used.

```
$ time git clone https://git.netfilter.org/nftables
...
real    1m16.848s
user    0m4.867s
sys     0m1.883s

$ time git clone git://git.netfilter.org/nftables
...
real    0m2.453s
user    0m1.180s
sys     0m0.158s
```

According to [1], there is a more modern "smart" protocol, which is
relatively more stable and fast.  However it's not supported by the
remote server, so let's use git:// as a workaround.

[1] https://www.git-scm.com/docs/http-protocol

Change-Id: I8e943c74052cc74eae8dc1d80ab243f792a90156

contrib/jenkins.sh

@@ -47,7 +47,7 @@ build_from_netfilter() {
 	if [ -d "./$project" ]; then
 		rm -rf "./$project"
 	fi
-	git clone "https://git.netfilter.org/$project" "$project"
+	git clone "git://git.netfilter.org/$project" "$project"
 	cd "$project"
 	autoreconf --install --force
 	./configure \