tools/resolve_btfids: Add /libbpf to .gitignore

stable inclusion
from stable-5.10.29
commit f890246ae75c4b21e1cd4d52a148b6145ca971f0
bugzilla: 51789

--------------------------------

[ Upstream commit 90a82b1f ]

This is what I see after compiling the kernel:

 # bpf-next...bpf-next/master
 ?? tools/bpf/resolve_btfids/libbpf/

Fixes: fc6b48f6 ("tools/resolve_btfids: Build libbpf and libsubcmd in separate directories")
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210212010053.668700-1-sdf@google.com


Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Chen Jun <chenjun102@huawei.com>
Acked-by:   Weilong Chen <chenweilong@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>