qt: disable precompiled headers to workaround compilation failure

Precompiled header implementation is buggy in current Qt as described at
http://comments.gmane.org/gmane.comp.lib.qt.devel/5933.

The only known workaround is currently to add -no-pch to configure.

Fixes build failure terminating with error
  compiling ../3rdparty/pixman/pixman-arm-neon-asm.S
  cc1: fatal error: .pch/release-shared-emb-arm/QtGui: No such file or directory
  compilation terminated.
such as:
http://autobuild.buildroot.net/results/648265012591280fa592a239f86ee39aec2d5fa6/build-end.log



Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>