...after 119d8137 "Revert 'Allow compiler
plugins for online'", to fix cases like

> In file included from vcl/unx/gtk3_kde5/gtk3_kde5_cairo.cxx:20:
> vcl/unx/gtk3_kde5/../gtk3/cairo_gtk3_cairo.cxx:12:10: error: replace "..." include form with <...> for inclusion of a source file not next to the current source file, or a header, vcl/unx/gtk3/cairo_gtk3_cairo.hxx [loplugin:includeform]
> #include "cairo_gtk3_cairo.hxx"
>          ^~~~~~~~~~~~~~~~~~~~~~

Change-Id: I12b3049862825f10834333b154c9343a3e3bec6d