qt: disable precompiled headers when ccache is enabled

ccache and precompiled headers don't work together, so disable
precompiled headers when ccache is enabled.

The benefit of ccache compiling Qt a second or more time is much
higher than the costs of disabled precompiled headers in the first run.

Signed-off-by: Michael Roth <mroth@nessie.de>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>