zlib: rename z_errmsg symbol to avoid name clash when linking statically

Task-number: QTBUG-35301
Change-Id: I298fba7270ee49d4e2dfc624b18aa13ce25864be
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: aavit <eirik.aavitsland@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>