Remove a forgotten instance of @libsuffix@

* lang/cpp/src/GpgmeppConfig.cmake.in.in: Remove a forgotten
instance of @libsuffix@.

000376ef9377fa45eef52bc50efd68babbf8680f removed @libsuffix@ from
cmake config files, but missed one instance.

• Signed-off-by: Heiko Becker <heirecka@exherbo.org>