in_ebpf: Prepare build directory for building eBPF objects

[cosmo0920]

Without the generated directory creation before compiling eBPF objects, I get failed to compile eBPF objects for in_ebpf plugin.
After adding that creation of directory, I succeeded to compile in_ebpf plugin in Ubuntu 24.04 with 6.11 (noble-proposed) Kernel and its associated linux-tools.

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