With just -include a.hxx ccache checks only for presence of a.hxx.gch
and it doesn't search the path given by -I, so it didn't detect the usage
of the .gch and thus didn't include it in the checksum, possibly leading
to false positives. Icecream similarly doesn't search paths given by -I
and may fail to properly handle the .gch usage.

Change-Id: I40ba2d5089e77cd5e8da670c7e030f9e90ebc8ac
Reviewed-on: https://gerrit.libreoffice.org/72703
Tested-by: Jenkins
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>