gen_makefile.py: use s#git://#https://# in git clone base URL

Unencrypted git:// protocol offers no integrity or authentication,
making it subject to tampering.  Use https:// instead.

https://github.blog/2021-09-01-improving-git-protocol-security-github/#no-more-unauthenticated-git
https://blog.readthedocs.com/github-git-protocol-deprecation/

Using https:// is also required for some new repositories (like
libosmo-gprs.git), which are intentionally unavailable over git://.

Change-Id: I503f0e1b5258102d771597f1b5b753f670832fcb

gen_makefile.py

@@ -70,8 +70,8 @@ parser.add_argument('-b', '--build-dir', dest='build_dir',
   help='''Parent dir for all build trees (default:
 directly in the make-dir).''')
 
-parser.add_argument('-u', '--url', dest='url', default='git://git.osmocom.org',
-  help='''git clone base URL. Default is 'git://git.osmocom.org'.
+parser.add_argument('-u', '--url', dest='url', default='https://git.osmocom.org',
+  help='''git clone base URL. Default is 'https://git.osmocom.org'.
 e.g. with a config like this in your ~/.ssh/config:
   host go
   hostname gerrit.osmocom.org