borgbackup: Fix build on i586

borgbackup: Fix build on i586

Fixes

...
'/usr/src/config/rootfiles/packages//borgbackup' -> '/install/packages/package/ROOTFILES'
tar: usr/lib/python3.6/site-packages/borg/chunker.cpython-36m-i586-linux-gnu.so: Cannot stat: No such file or directory
tar: usr/lib/python3.6/site-packages/borg/compress.cpython-36m-i586-linux-gnu.so: Cannot stat: No such file or directory
tar: usr/lib/python3.6/site-packages/borg/crypto.cpython-36m-i586-linux-gnu.so: Cannot stat: No such file or directory
tar: usr/lib/python3.6/site-packages/borg/hashindex.cpython-36m-i586-linux-gnu.so: Cannot stat: No such file or directory
tar: usr/lib/python3.6/site-packages/borg/platform_linux.cpython-36m-i586-linux-gnu.so: Cannot stat: No such file or directory
tar: Exiting with failure status due to previous errors
make: *** [borgbackup:58: dist] Error 2
...

Best,
Matthias

Signed-off-by: Matthias Fischer <matthias.fischer@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>