kbuild, bpf: Correct pahole version that supports distilled base btf feature

pahole commit [0] of supporting distilled base btf feature released on
pahole v1.28 rather than v1.26. So let's correct this.

Signed-off-by: Pu Lehui <pulehui@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://git.kernel.org/pub/scm/devel/pahole/pahole.git/commit/?id=c7b1f6a29ba1 [0]
Link: https://lore.kernel.org/bpf/20250219063113.706600-1-pulehui@huaweicloud.com