Pre-compiled headers should not be enabled by default when using ccache

I tried to build Qt Creator with qbs and ccache. In qmake ccache does provide a significant speedup, with Qbs there is hardly any cache hit, so build times are almost identical with ccache and without (actually with ccache they are slightly worse due to ccache's own overhead).

Disabling precompiled headers makes qbs hit caches and speeds up build times significantly.