Fix MSVC link warning on TriggerEx

[Brett208]

When building HFL, I get the following warning:

Warning LNK4221 This object file does not define any previously undefined public symbols, so it will not be used by any link operation that consumes this library HFL \LevelTemplate\OP2MissionSDK\HFL\Source\TriggerEx.obj 1

[DanRStevens]

Hmm, and that's exactly the kind of empty source file I just added to Outpost2DLL, that only includes a header file. Very interesting that the compiler warns about that. Curiously, AppVeyor doesn't show any warnings for Outpost2DLL for the empty implementation file.

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Here, the file contains the comment: // TODO: TriggerEx implementation
Perhaps the solution is the implement the class defined in TriggerEx.h.