Copy libgcrypt-20.dll and libgpg-error-0.dll into your  Program Files (x86)\Pidgin\  folder

Copy lurch.dll OR lurch-compat.dll (but not both) into your  Program Files (x86)\Pidgin\plugins  folder

Use lurch.dll to be compatible with the XEP (XMPP specification)
Use lurch-compat.dll to be compatible with other OMEMO clients, breaking spec