Add DDS_TIME_INVALID and DDS_Time_is_invalid for connext micro

[cwecht]

For whatever reasons ConnextDDS micro exports DDS_TIME_INVALID but it doesn't define it. Therefore this results in a runtime error - because during runtime as the shared library is loaded, the symbol to DDS_TIME_INVALID can not be resolved.

Also DDS_Time_is_invalid is not defined. So both need to be added in order to make rmw_connextmicro work.

[cwecht]

This PR is basically a direct follow up to #120. At the time I only checked whether rmw_connextmicro build successfully. Later I discuvered this issue with Connext Micro exporting symbols which it doesn't define.

@asorbini it would be great if you could have a look at this.

[fujitatomoya]

@cwecht it really looks like the problem is with ConnextDDS micro where it should address your issue here? Having the work-around to bypass the problem would be unexpected behavior in the future, since all these definition need to be in the DDS implementation?

@asorbini it would be appreciated if you can chime in here.

[cwecht]

@asorbini I found neither for ConnextDDS Micro 3.0.3.28 nor for 2.4.13.4 any definition for DDS_TIME_INVALID.