Add FindLibGpgError.cmake to find Libgpg-error

Use a proper FindLibGpgError CMake module. The old FindAssuan2 also looked
for Libgpg-error, but the new one doesn't.

We require Libgpg-error 1.36 which is the same version required by the
version of GpgME we require.

• GnuPG-bug-id: T6310