Add workaround for 'bpf_task_work' on Linux 6.18

[ekyooo]

Fix compilation errors caused by bpf_task_work being forward-declared in Linux 6.18. A placeholder definition is added to virtual_bpf.h for compatibility.

Related to libbpf commit f820b82.

[arges]

It may be helpful to ifndef around the structure to ensure it doesn't get defined more than once. For example:

#ifndef __BPF_TASK_WORK_DEFINED__
#define __BPF_TASK_WORK_DEFINED__
struct bpf_task_work {
       __u64 __opaque;
} __attribute__((aligned(8)));
#endif

[ekyooo]

It may be helpful to ifndef around the structure to ensure it doesn't get defined more than once. For example:

@arges
Thanks for the feedback. This PR has been superseded by the following one:
#5455