Fix Win32 warning C4702: unreachable code

..by disabling the warning for the moment. This happens on x86 builds
with VS2017, but only with OSL_DEBUG_LEVEL=0, e.g. at:

 cppuhelper/source/servicemanager.cxx(1311) : error C2220: warning treated as error - no ´object´ file generated
 cppuhelper/source/servicemanager.cxx(1311) : warning C4702: unreachable code
 cppuhelper/source/servicemanager.cxx(1373) : warning C4702: unreachable code

Change-Id: I91943350d7f6f5acfedb3b2bb14e5d835e4494ed
Reviewed-on: https://gerrit.libreoffice.org/57472Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>
Tested-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>

solenv/gbuild/platform/WNT_INTEL_MSC.mk

@@ -22,6 +22,9 @@ gb_CPUDEFS := -D_X86_=1
 
 include $(GBUILDDIR)/platform/com_MSC_defs.mk
 
+gb_CXXFLAGS += \
+        $(if $(filter 0,$(gb_DEBUGLEVEL)),-wd4702) \
+
 include $(GBUILDDIR)/platform/com_MSC_class.mk
 
 # vim: set noet sw=4: