abilint: fix return types bool -> int

abilint: fix return types bool -> int

Returning bool literals from main can be misleading. Returning booleans
maps to (by convention):

   return false -> converted to 0 -> rc=0 considered SUCCESS
   return true  -> converted to 1 -> rc=1 considered FAILURE

Compiling with clang also emits:

  abilint.cc:258:7: warning: bool literal returned from 'main' [-Wmain]
      return true;
      ^      ~~~~

The issues can be addressed by consistently returning integers as also
done in all other mains across the project.

Same issue applies to print-diff-tree.cc.

 * tools/abilint.cc: return int in main rather than bool.
 * tests/print-diff-tree.cc: Likewise.

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