You can create New Toolbar Button plugin which is shipped with error:
\Button\Public\ButtonStyle.h(8): error C2011: ‘FButtonStyle’: ‘struct’ type redefinition
Just create new Toolbar button Plugin (using View->plugins->new plugin button) and try to recompile.
I tried to reproduce this in 4.12.5 (both the binary version as well as when built from source code), and was unable to see the error that you mentioned after adding a new Toolbar button plugin. Would you be able to elaborate on the steps you are following when you add the plugin?
We have not heard back from you for a while. Do you still need help with this issue? I will be marking this post as resolved for internal tracking purposes, but please feel free to add a comment and re-open the post if you need any additional assistance.