xmlsecurity: fix --without-system-nss build

signing2.cxx:71: error: undefined reference to 'NSS_OptionSet'

(regression from commit f0efcb9170)

Change-Id: Idad3f7d5d1a2cb642ac3485a750cdc7631fbf4df
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/179954
Reviewed-by: Michael Stahl <michael.stahl@allotropia.de>
Tested-by: Jenkins

xmlsecurity/CppunitTest_xmlsecurity_signing2.mk

@@ -42,6 +42,7 @@ ifneq ($(OS),WNT)
 ifneq (,$(ENABLE_NSS))
 $(eval $(call gb_CppunitTest_use_externals,xmlsecurity_signing2,\
     nssutil3 \
+    nss3 \
 ))
 endif
 endif