softoff: Fix sdeventplus exception

error link:
https://jenkins.openbmc.org/job/CI-MISC/job/ci-repository-ppc64le/1255/console

Signed-off-by: George Liu <liuxiwei@inspur.com>
Change-Id: If5db9c421df502348e6fdf6e64c46c8cdf02e758
diff --git a/softoff/softoff.cpp b/softoff/softoff.cpp
index 890b4af..d215c1d 100644
--- a/softoff/softoff.cpp
+++ b/softoff/softoff.cpp
@@ -11,6 +11,7 @@
 
 #include <sdbusplus/bus.hpp>
 #include <sdeventplus/clock.hpp>
+#include <sdeventplus/exception.hpp>
 #include <sdeventplus/source/io.hpp>
 #include <sdeventplus/source/time.hpp>