Previously the function signature was inconsistent throughout
src/fe-gtk/plugin-notification.{h,c}: One file had the signature
int(void), while the other had int(void *). Since this type mismatch
might lead to problems (especially with LTO) and the (possibly provided)
function argument isn't used in the function's definition, this commit
sets int(void *) as function signature for both the declaration and
definition of the function.
Fixes: https://github.com/hexchat/hexchat/issues/2726
