Disable headers precompilation

qtbase's cb09e1e enables precompiled headers by default.
Unfortunately qmake tries to precompile them as C headers
as a dependency of the .c protocol files we have, failing
the build.
Quick and easy solution: disable them alltogether.

Change-Id: I96a4f4fd1fe0d3a83bb990408f1f3994650d96c5
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>