Ignore zero valued enum entries from badBitmaskCheck

[mptre]

Usage of zero valued enum entries can be used for documenting purposes and should be ignored just like zeroes expanded from macros.

[chrchr-github]

I have no strong opinion on this, but the difference is that macros might be configuration-dependent. One can easily imagine a bug where an enumerator with value 0 is used with | instead of the correct one.
enum E { CONFUSING11, CONFUSING111 };

[danmar]

One can easily imagine a bug where an enumerator with value 0 is used with | instead of the correct one.

yes it seems that there are many findings in 2.11 that are not found in latest HEAD:
http://cppcheck1.osuosl.org:8000/diff-badBitmaskCheck

I have checked 5-6 random warnings in erlang, far2l, exim.. and all those looked intentional (true negatives) to me. At least I did not see other suspicious enum constants with similar names that could have been intended instead.