UE FileMediaSource Forward Declare issue

Note for self or other: The FileMediaSource being forward declared in a struct was causing an error at build, a type error:
"error C4430: missing type specifier - int assumed. Note: C++ does not support default-int
error C2146: syntax error: missing ';' before identifier 'UClass'
error C2065: 'Z_Construct_UClass_UFileMediaSource_NoRegister': undeclared identifier"

To fix the issue it was needed to add a dependency to the plugin's (or project's) Build.cs.
For the case of the FileMediaSource adding FileMediaSource or MediaAssets fixes the issue.