desktop: enable CppunitTest_desktop_lib only on Linux

Since the test would fail to link on Windows, due to

ifeq ($(GUIBASE),unx)
$(eval $(call gb_Library_add_exception_objects,sofficeapp,\
    desktop/source/lib/init \
))
endif

in Library_sofficeapp.mk. Given that
CppunitTest_libreofficekit_tiledrendering is marked as Linux-only as
well, be consistent and have this test as Linux-only, too.

Change-Id: I6c8884398eba5dcf8a74c9cdc96c869b6108fb7e
(cherry picked from commit ac882c30)

desktop/Module_desktop.mk

@@ -130,8 +130,10 @@ $(eval $(call gb_Module_add_check_targets,desktop, \
     CppunitTest_desktop_version \
 ))
 
+ifeq ($(OS),LINUX)
 $(eval $(call gb_Module_add_check_targets,desktop, \
     CppunitTest_desktop_lib \
 ))
+endif
 
 # vim: set ts=4 sw=4 et: