Update Makefile with correct libgit2 repo url

gdk update was raising errors upon attempt to reach libgit2 repository

Building gitlab-org/gitaly ba63ee19fb2dafe6f2ca5bca5d12a0b24837ce17
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fatal: unable to access 'https://gitlab.com/libgit2/libgit2/': The requested URL returned error: 403
make[2]: *** [/Users/mikolajwawrzyniak/gitlab/gitlab-development-kit/gitaly/_build/deps/libgit2/install/lib/libgit2.a] Error 128
make[2]: *** Waiting for unfinished jobs....
make[1]: *** [/Users/mikolajwawrzyniak/gitlab/gitlab-development-kit/gitaly/_build/bin/gitaly] Error 2
make: *** [gitaly-update-timed] Error 2
❌️ ERROR: Failed to update.

This MR updates libgit2 repository