button-handler: use sdeventplus loop

Use the sdeventplus event loop so that an upcoming commit can make use
of sdeventplus timers.

Change-Id: I596ee36141f16ceb5e95078bad9fbe6a12c832a6
Signed-off-by: Matt Spinler <spinler@us.ibm.com>
diff --git a/src/button_handler_main.cpp b/src/button_handler_main.cpp
index e71f8e4..7c21fe9 100644
--- a/src/button_handler_main.cpp
+++ b/src/button_handler_main.cpp
@@ -1,15 +1,15 @@
 #include "button_handler.hpp"
 
+#include <sdeventplus/event.hpp>
+
 int main(void)
 {
     auto bus = sdbusplus::bus::new_default();
+    auto event = sdeventplus::Event::get_default();
+
+    bus.attach_event(event.get(), SD_EVENT_PRIORITY_NORMAL);
 
     phosphor::button::Handler handler{bus};
 
-    while (true)
-    {
-        bus.process_discard();
-        bus.wait();
-    }
-    return 0;
+    return event.loop();
 }