button-handler:  Add button-handler service file

This service will start the button-handler app on the way to
standby after the mapper and the buttons app.

Tested:  Built an image and saw it start.

Change-Id: I4d08fa64f2a15486a4ccd33eaf7c6a89a281594e
Signed-off-by: Matt Spinler <spinler@us.ibm.com>
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 0fa7a58..2f631b7 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -71,6 +71,7 @@
 set (
     SERVICE_FILES
     ${PROJECT_SOURCE_DIR}/service_files/xyz.openbmc_project.Chassis.Buttons.service
+    ${PROJECT_SOURCE_DIR}/service_files/phosphor-button-handler.service
 )
 
 install (FILES ${SERVICE_FILES} DESTINATION /lib/systemd/system/)