Doesn't really matter if you use `""` or `<>`, the only difference is that `""` first looks in the location of the source file that is being compiled (I believe it's actually the working directory of the compiler), whereas `<>` looks only in paths provided on the command line and default include dirs (/usr/include) (`""` also looks in these paths).
So for geanyplugin.h you need the include path on the command line, so both `<>` and `""` are equally correct. But, by convention, you should prefer `<>` for headers that don't belong to your package, which is the case here.