bpf: add a comment that bpf_trace_printk() is GPL only am: 3935d8b06c am: 4ada620e20

Original change: https://android-review.googlesource.com/c/platform/packages/modules/Connectivity/+/3255319

Change-Id: Ife29b975798699de3aeb03e31aaa360a184c5a14
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>