Some kernels (e.g. 5.4 with backports) include proc_ops despite being
older than 5.6.0. Replace hardcoded version check with runtime header
detection to handle these cases.
- Check for "struct proc_ops" in include/linux/proc_fs.h
- Use KSU_COMPAT_HAS_PROC_OPS macro for conditional compilation
- Fixes build failures on kernels with backported proc_ops
Signed-off-by: JackAltman
