Disable compiler warnings for libqtharfbuzz-ng

This 3rdparty code produces a lot of warnings especially with clang.

Change-Id: I6f48410699e785d1b2e84b9a6d7b0ba8751179b0
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>