Remove NTPServers duplicate values and null values

When saving the set NTPServers values from webUI, NTPServer may contain
duplicate values and null values and update them to D-Bus.

Now, need to parse and verify the value of the ntpServers attribute,and
remove duplicate values and null values.

Tested:save NTP and check it via D-Bus
without this patch:
NTPServers  property  as  3 "" "10.164.29.2" "10.164.29.2"

with this patch:
NTPServers  property  as  2 "" "10.164.29.2"

Signed-off-by: George Liu <liuxiwei@inspur.com>
Change-Id: I52291e4608efd635b179f3934c3d3e805afd2209
diff --git a/meson.build b/meson.build
index 7673112..863cad1 100644
--- a/meson.build
+++ b/meson.build
@@ -359,6 +359,7 @@
   'redfish-core/ut/lock_test.cpp',
   'redfish-core/ut/configfile_test.cpp',
   'redfish-core/ut/time_utils_test.cpp',
+  'redfish-core/ut/stl_utils_test.cpp',
   'redfish-core/ut/hex_utils_test.cpp',
   'http/ut/utility_test.cpp'
 ]