Always define _R__UNIQUE_DICT_.

Since it is now different from _R__UNIQUE_ and used explicit, we can define it
unconditionally and thus avoid the case (used by MakeProject) where the header
is parsed before R__DICTIONARY_FILENAME is declared (technically this means
that previously the dictionary generated by MakeProject _could_ have had
some naming clashes with similar dictionary since the _R__UNIQUE_ used was
the non-dictionary one).