Add "gui_vimr" feature to has_list

VimR requires that macvim be built with both FEAT_GUI_MACVIM and
FEAT_GUI_VIMR, though in this case only "gui_vimr" is added to
the has_list. It may make sense to add a common feature in the future,
like FEAT_GUI_MACVIM_FRAMEWORK/"gui_macvim_framework".