glsl: get correct member type when processing xfb ifc arrays

This fixes a crash in:

KHR-GL45.enhanced_layouts.xfb_block_stride

Fixes: 08225179 "glsl: add helper to process xfb qualifiers during linking"
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
(cherry picked from commit 9d53cccc)