scripts/bpf_doc: Use silent mode when exec make cmd

When getting kernel version via make, the result may be polluted by other
output, like directory change info. e.g.

  $ export MAKEFLAGS="-w"
  $ make kernelversion
  make: Entering directory '/home/net'
  6.8.0
  make: Leaving directory '/home/net'

This will distort the reStructuredText output and make latter rst2man
failed like:

  [...]
  bpf-helpers.rst:20: (WARNING/2) Field list ends without a blank line; unexpected unindent.
  [...]

Using silent mode would help. e.g.

  $ make -s --no-print-directory kernelversion
  6.8.0

Fixes: fd0a38f9

 ("scripts/bpf: Set version attribute for bpf-helpers(7) man page")
Signed-off-by: Michael Hofmann <mhofmann@redhat.com>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Quentin Monnet <qmo@kernel.org>
Acked-by: Alejandro Colomar <alx@kernel.org>
Link: https://lore.kernel.org/bpf/20240315023443.2364442-1-liuhangbin@gmail.com