CPackDEB: Use proper compression scheme for control.tar.gz

Changes in commit v3.4.0-rc1~79^2~1 (CPackDeb: use of libarchive and
removal of fakeroot, 2015-09-11) accidentally set the wrong compression
scheme for the `control.tar.gz` file.  Set it explicitly to GZip.

Source/CPack/cmCPackDebGenerator.cxx

@@ -565,7 +565,7 @@ int cmCPackDebGenerator::createDeb()
       return 0;
       }
     cmArchiveWrite control_tar(fileStream_control_tar,
-                               tar_compression_type,
+                               cmArchiveWrite::CompressGZip,
                                "paxr");
 
     // sets permissions and uid/gid for the files