move syncqt call to qt_module_headers.prf

move syncqt call to qt_module_headers.prf

for one, the syncqt invocation is qt module magic and thus simply does
not belong into the generic default_pre file.
second, this way the forwarding header generation is now linked to the
rest of the build magic for a particular module, which is way less
confusing for the unsuspecting developer.

Change-Id: Idc8e420d3faf173d7fff4a41e6e1c59af15c3023
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>