tools/bpf: Add bootstrap/ to .gitignore

Commit 8859b0da

 ("tools/bpftool: Fix cross-build") added a
build-time bootstrap/ directory for bpftool, and removed
bpftool-bootstrap. Update .gitignore accordingly.

Reported-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20201112091049.3159055-1-jean-philippe@linaro.org