It a) looked a bit silly to use .gch even with MSVC, b) allows to build
something with Clang and GCC without them trying to use each other's PCH
file, c) is nitpicking, yeah.

Change-Id: I478beeb6aab6378cbdcc6ea75aa2e3ff2bcf6f87
Reviewed-on: https://gerrit.libreoffice.org/71582
Tested-by: Jenkins
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>