tools: Work around BITS_PER_LONG related build failure in objtool

The objtool build fails with the recent changes to the bits-per-long
headers:

  tools/include/linux/bitops.h:12:0: error: "BITS_PER_LONG" redefined [-Werror]

Which got introduced by:

  bb970707 tools: Copy the bitsperlong.h files from the kernel

Work it around for the time being.

Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>