www/{*chromium,iridium}: xmlError is only a const since version 2.12

the previous patches were checking if libxml is older than 2.11.6,
but that breaks builds with 2.11.7 as xmlError has been changed to
a const at 2.12

PR: 276901, 276902, 276903
Reported by: diizzy@

(cherry picked from commit 1e1b3d42f560254a9c6e533d20e1c081c184cf5b)