updated weechat-plugin.h

[sum-catnip]

I went ahead and updated the weechat-plugin header since weechat was complaining.
I was able to compile and run the plugin with weechat.

[sjrct]

I ran into a similar issue, but it was that my weechat version was older than the weechat-plugin.h in this repo (Ubunutu packages are slow to update...). What about using a system-installed weechat-plugin.h? Certainly much more likely to correspond to the user's proper weechat version