Revert "Makefile.am: Do not ship generated grammar files"

This reverts commits cb12ba8ef1fcac6b789485e9c8381ea04c59fda2 and
6872bd6360689e42875535096b5049f67ba23367.
Makefile.am: Do not ship generated grammar files, properly

This patch prevents make dist from shipping generated grammar files,
doing so in a more portable, reliable way than the previous, now
reverted patches.

Reported-by: Peter Czanik <czanik@balabit.hu>
Signed-off-by: Gergely Nagy <algernon@balabit.hu>