manager: Switch to lambda

Slightly more concise syntax and also interoperable with function2

Change-Id: If5b33f5b8da65680bdf5b9cd3c43579b1cf126f2
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/manager.cpp b/manager.cpp
index 5f23208..88406f4 100644
--- a/manager.cpp
+++ b/manager.cpp
@@ -27,8 +27,7 @@
     using namespace sdbusplus::bus::match::rules;
     settingsMatches.emplace_back(
         bus, propertiesChanged(settings.timeSyncMethod, settings::timeSyncIntf),
-        std::bind(std::mem_fn(&Manager::onSettingsChanged), this,
-                  std::placeholders::_1));
+        [&](sdbusplus::message_t& m) { onSettingsChanged(m); });
 
     // Check the settings daemon to process the new settings
     auto mode = getSetting(settings.timeSyncMethod.c_str(),