test/Makefile: Add sdbeventplus to LDFLAGS

Commit eb5d3f2fbcf584a28dc7aeaed050243ba0a038ea added a new
function that uses sdeventplus. This causes the test cases
to fail with:
undefined reference to `sdeventplus::Event::get_default().
Add the sdeventplus to LDFLAGS to fix it.

Tested: CI passes.

Change-Id: I6a5a79f15fc766be50ff78d9ee9d18e6380ef4d7
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
diff --git a/test/Makefile.am b/test/Makefile.am
index b14c0ae..d054a4c 100644
--- a/test/Makefile.am
+++ b/test/Makefile.am
@@ -41,6 +41,7 @@
 	$(OESDK_TESTCASE_FLAGS) \
 	$(PHOSPHOR_DBUS_INTERFACES_LIBS) \
 	$(SDBUSPLUS_LIBS) \
+	$(SDEVENTPLUS_LIBS) \
 	$(SYSTEMD_LIBS) \
 	$(CODE_COVERAGE_LIBS)