main/libevent: fix shebang in /usr/bin/event_rpcgen.py

Before this change:

$ dabuild sh
$ sudo apk add libevent-dev >/dev/null && event_rpcgen.py /dev/null
env: can't execute 'python': No such file or directory

After this change:

$ dabuild sh
$ sudo apk add libevent-dev >/dev/null && event_rpcgen.py /dev/null
Reading "/dev/null"
✂