gpio: Fix missing cassert

This file is relying on assert() being transitively provided by
sdbusplus. Fix the includes.

Change-Id: Ia081478eb55a90bf5af541a4b16124dcfb224a13
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/gpio-util/gpio.cpp b/gpio-util/gpio.cpp
index da5801e..c6b7704 100644
--- a/gpio-util/gpio.cpp
+++ b/gpio-util/gpio.cpp
@@ -18,6 +18,7 @@
 #include <fcntl.h>
 #include <sys/ioctl.h>
 
+#include <cassert>
 #include <phosphor-logging/log.hpp>
 
 namespace phosphor