Revert "neighbour: Replace zero-length array with flexible-array member"

This reverts commit 314713ff which is
commit 08ca27d0 upstream.

It breaks the CRC generation of symbols, and doesn't do any functional
changes, so is safe to revert.

Bug: 161946584
Change-Id: I26157e0e928e2b6d60e0acafc892efdad13c12b4
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>