Fix thai symbol rendering slowness

Harfbuzz is constantly trying to load libthai in
src/3rdparty/harfbuzz/src/harfbuzz-thai.c,
making rendering of thai symbols very slow.

Task-number: QTBUG-37665
Change-Id: I30a2a5489271534eeacc205d8106b22130c40c5f
(cherry picked from qtbase/47e6ec0736aa129e181e0b6f02cec884f9832432)
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>