add "system" PCH that contains often used system headers such as STL

The idea is that this PCH will be used as a fallback for linktargets
that do not explicitly set their own PCH.

Change-Id: If05d3f04893a386e5d991f913e094f49249c8f44
Reviewed-on: https://gerrit.libreoffice.org/79363
Tested-by: Jenkins
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>

RepositoryModule_host.mk

@@ -97,6 +97,7 @@ $(eval $(call gb_Module_add_moduledirs,libreoffice,\
 	oox \
 	$(call gb_Helper_optional,OPENCL,opencl) \
 	package \
+	pch \
 	postprocess \
 	$(call gb_Helper_optional,PYUNO,pyuno) \
 	$(call gb_Helper_optional,QADEVOOO,qadevOOo) \