ci: add empty ebpf build-type patch dir so the ebpf flavor builds

build-kernel.sh applies build-type patches from
patches/<ver>.x/<build_type>, and apply_patches.sh treats a missing
directory as a fatal error. The new patches/ dir (added with the fs/dax
patch) had no ebpf subdir, so the ebpf matrix flavor failed at 'Setup
kernel source' while base (empty build_type) passed. The ebpf flavor has
no source patches of its own, so add a no_patches.txt sentinel dir.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: adam-rl

packaging/kernel/patches/6.18.x/ebpf/no_patches.txt

@@ -0,0 +1,8 @@
+The ebpf build type has no kernel source patches of its own.
+
+The eBPF/BTF tracing stack is enabled entirely through config fragments
+(see packaging/kernel/build-type/ebpf), not source patches. Version-level
+patches in the parent directory (6.18.x/) still apply to this build type.
+
+This file exists so apply_patches.sh finds a directory here and treats it
+as "no patches to apply" instead of failing. See packaging/scripts/apply_patches.sh.