Disable NSS GTESTS also for Windows

Following https://cgit.freedesktop.org/libreoffice/core/commit/?id=02623af4f5e0049ec5988df8ff6c7ff0f32f3fb1
See details here:
http://document-foundation-mail-archive.969070.n3.nabble.com/fatal-error-LNK1561-on-nss-Visual-Studio-2017-td4256601.html

Change-Id: I75f2c8a5409e484a8a5a12a6f12d5c20b244d2b2
Reviewed-on: https://gerrit.libreoffice.org/67092
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>

external/nss/ExternalProject_nss.mk

@@ -25,6 +25,7 @@ $(call gb_ExternalProject_get_state_target,nss,build): $(call gb_ExternalExecuta
 		XCFLAGS="-arch:SSE $(SOLARINC)" \
 		$(MAKE) -j1 nss_build_all RC="rc.exe $(SOLARINC)" \
 			NSINSTALL='$(call gb_ExternalExecutable_get_command,python) $(SRCDIR)/external/nss/nsinstall.py' \
+			NSS_DISABLE_GTESTS=1 \
 	,nss)
 
 else # OS!=WNT