Added public compile definitions to TrigConfData.

Normally, when using an "offline build", those flags get set up project-wide.
But Point-1 projects only include the definitions of the exported targets,
without the use of the DetCommon project files. For such downstream projects
to work without hardcoding how they should use these exported targets, the
XAOD_STANDALONE and TRIGCONF_STANDALONE compiler flags are put explicitly
on the TrigConfData library.

Much of the rest of the DetCommon libraries (that care about these flags
anyway...) depend on TrigConfData, so I didn't add this anywhere else.