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

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

Change-Id: If1f97466b49ff3c911bb3dbd52c2c0cd69e86029
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>