Oops, this reference can't be const.

It's used in ImplFontCache::GetGlyphFallbackFont.

This is fix for commit a6b00d16eb27a5e7e31c721671001a909ecef960,
which caused huge performance regression for document in fdo#59882,
fortunately immediately detected by loperf.

Change-Id: I475742b5249f106d34c4f6c43b1e39e9bb7b897a