mainloop: Include <cassert>

We are currently relying on sdbusplus implicitly including cassert for
us. Since we use assert(), depend on the header providing it.

Change-Id: Idb771e74d5c0d5bdb0d82455dfd9d2e1a8b6b6ef
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/mainloop.cpp b/mainloop.cpp
index 4db0940..29bd378 100644
--- a/mainloop.cpp
+++ b/mainloop.cpp
@@ -28,6 +28,7 @@
 #include "targets.hpp"
 #include "thresholds.hpp"
 
+#include <cassert>
 #include <cstdlib>
 #include <functional>
 #include <iostream>