ZNC header files in /usr/local/include/znc/

Building ZNC from https://github.com/znc/znc puts the include files in /usr/local/include/znc/, so an attempt to build this plugin looked like so:
```
$ znc-buildmod cmdnotify.cpp
Building "cmdnotify.so" for ZNC 1.1... cmdnotify.cpp:14:17: error: znc.h: No such file or directory
cmdnotify.cpp:15:18: error: Chan.h: No such file or directory
cmdnotify.cpp:16:18: error: User.h: No such file or directory
cmdnotify.cpp:17:24: error: IRCNetwork.h: No such file or directory
cmdnotify.cpp:18:21: error: Modules.h: No such file or directory
...
```
By prepending the `znc/` in the includes as I've done in the commit the plugin was built with no errors. I'm not suggesting that this is the fix for everyone, since it would break existing builds, but I don't know enough about C++ to offer a more robust solution.