Pristine tar 4.12.3

Here is a pristine-tar branch for 4.12.3.

$ sha256sum samba_4.12.3+dfsg.orig.tar.xz
0872b3a710c9b5471f231f2673f3b33d6e2e15e8aece0fa5f4b38c4572b96011  samba_4.12.3+dfsg.orig.tar.xz