build: actually use CLOCK_GETTIME_LIBS for libzunitc

The fix/check in 34d59859 is incomplete.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
diff --git a/Makefile.am b/Makefile.am
index c042c68..41f0789 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -1054,7 +1054,8 @@
 	-I$(top_srcdir)/tools/zunitc/inc
 
 libzunitc_la_LIBADD = \
-	libshared.la
+	libshared.la \
+	$(CLOCK_GETTIME_LIBS)
 
 if ENABLE_JUNIT_XML
 libzunitc_la_CFLAGS += \