libbpf: Fix KERNEL_VERSION macro

Add missing ')' for KERNEL_VERSION macro.

Signed-off-by: Hengqi Chen <hengqi.chen@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210405040119.802188-1-hengqi.chen@gmail.com