Linker errors in 5.6 due to moving the use of _API defines from types to individual methods

Development Pipeline & Plugins
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Steps to Reproduce
Have a project in 5.5 that uses a previously exposed method from a separate plugin, such as `UPCGSplineProjectionData::GetSpline` (exported due to `PCG_API` on the parent class `UPCGProjectionData`), and then try to compile under 5.6. Linker errors will occur.