Fix build error on macOS: "printf: missing format character"

Fix build error on macOS: "printf: missing format character"

Commit 3306a802a11b8d "_helpers: fix module init, and get it at least
building with py3" breaks building bup on macOS:

  $ make
  [...]
  running build_ext
  find lib/bup/build/* -maxdepth 1 -name '_helpers*.so' \
            -exec printf 'x\c' '{}' \; | wc -c | xargs test 1 -eq
  printf: missing format character
  make: *** [lib/bup/_helpers.so] Error 1

So don't count characters printed for each file found, but count the
filenames found ("lines") itself.

Tested on Debian Linux 9 and macOS 10.13.3.