Fix gb_CppunitTest__use_vcl dependencies

Regression introduced with 41c66312 "The 'svp'
'plug-in' library is empty so don't build it at all"

Change-Id: I534173372ee21e16c78693f60957706bbf61c1c8

solenv/gbuild/CppunitTest.mk

@@ -174,7 +174,7 @@ $(call gb_CppunitTest_get_target,$(1)) : VCL := $(true)
 $(call gb_CppunitTest_get_target,$(1)) : $(call gb_Library_get_target,vclbootstrapprotector)
 ifeq ($(USING_X11),TRUE)
 $(call gb_CppunitTest_get_target,$(1)) : $(call gb_Library_get_target,desktop_detector)
-$(call gb_CppunitTest_get_target,$(1)) : $(if $(filter $(2),$(true)), \
+$(call gb_CppunitTest_get_target,$(1)) : $(if $(filter $(2),$(true)),, \
     $(call gb_Library_get_target,vclplug_gen) \
         $(if $(ENABLE_GTK),$(call gb_Library_get_target,vclplug_gtk)) \
         $(if $(ENABLE_GTK3),$(call gb_Library_get_target,vclplug_gtk3)) \