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/meson.build b/meson.build
index 4d1d370..dc55722 100644
--- a/meson.build
+++ b/meson.build
@@ -39,6 +39,7 @@
 )
 
 sdbusplus_dep = dependency('sdbusplus')
+sdeventplus_dep = dependency('sdeventplus')
 phosphor_dbus_interfaces_dep = dependency('phosphor-dbus-interfaces')
 phosphor_logging_dep = dependency('phosphor-logging')
 gpioplus_dep = dependency('gpioplus')
@@ -59,6 +60,7 @@
     phosphor_logging_dep,
     nlohmann_json_dep,
     gpioplus_dep,
+    sdeventplus_dep,
 ]
 
 sources_buttons = [