Include <memory> header in events.hpp

[marcelwa]

events.hpp makes heavy use of std::shared_ptr. Hence, it should include the <memory> header. Otherwise, one relies on the including file to include <memory> first. This lead to a compile error on my side, so I thought a fix might be helpful.

For me, https://include-what-you-use.org/ has proven helpful to avoid such issues.

[hriener]

Thanks!