Add common BPF helper to include path

All BPF programs need helper header files. These end up being
duplicated. Let us add a path to the common BPF helper to the search
path so that it can be used across projects.

No functional change to the final system image, only a build change.

Bug: 122665156
Change-Id: Ieee838df98258213efa24d74a12e069f056c66c1
Signed-off-by: Joel Fernandes <joelaf@google.com>