example/heartbeat: Add signal handling

This makes the example a little more interesting as we now have multiple
sources and we can analyze memory safety with valgrind by just sending a
SIGINT to quit cleanly.

Change-Id: Id15fe58f798d3b137f91228025f48a5e4cd5fc50
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/example/meson.build b/example/meson.build
index 1f18444..ea04ff5 100644
--- a/example/meson.build
+++ b/example/meson.build
@@ -9,5 +9,6 @@
   executable(example, example + '.cpp',
              include_directories: includes,
              implicit_include_directories: false,
-             link_with: sdeventplus)
+             link_with: sdeventplus,
+             dependencies: stdplus)
 endforeach