tests: eventtest: fix LDADD

Don't explicitly LDADD -lrt, use $(LIB_CLOCK_GETTIME) because
not all platforms have clock_gettime(2) and librt available.

tests/Makefile.am

@@ -1310,7 +1310,7 @@ endif WITH_DRIVER_MODULES
 if WITH_LIBVIRTD
 eventtest_SOURCES = \
 	eventtest.c testutils.h testutils.c
-eventtest_LDADD = -lrt $(LDADDS)
+eventtest_LDADD = $(LIB_CLOCK_GETTIME) $(LDADDS)
 endif WITH_LIBVIRTD
 
 libshunload_la_SOURCES = shunloadhelper.c