add missing virtual destructors

add missing virtual destructors

Several virtual desctructors were missing. Even though there might not
have been actual leaks or similar bugs, it is worth fixing these
locations as they might lead to bugs in the future.

Clang also warns at these locations:

  warning: delete called on non-final 'abigail::ir::corpus' that has virtual
  functions but non-virtual destructor [-Wdelete-non-virtual-dtor]

 * include/abg-comparison.h: add virtual destructor for corpus_diff and diff_node_visitor
 * include/abg-corpus.h: add virtual destructor for corpus
 * include/abg-reporter.h: add virtual destructor for reporter_base
 * include/abg-traverse.h: add virtual destructor for traversable_base

Signed-off-by: Matthias Maennich <maennich@google.com>