Fix compilation warning regarding std::variant

When building with SDK, the compiler complains about
’variant’ being not a member of ’std’:

.../pack.hpp:249:24: error: ‘variant’ is not a member of ‘std’

This commit adds appropriate #include to fix that.

Change-Id: I99d6b7c17cbe1f49d706821797cf3fa03ca8c26a
Signed-off-by: Alexander Amelkin <a.amelkin@yadro.com>
diff --git a/include/ipmid/message/pack.hpp b/include/ipmid/message/pack.hpp
index 4314ac1..18863c4 100644
--- a/include/ipmid/message/pack.hpp
+++ b/include/ipmid/message/pack.hpp
@@ -22,6 +22,7 @@
 #include <phosphor-logging/log.hpp>
 #include <tuple>
 #include <utility>
+#include <variant>
 #include <vector>
 
 namespace ipmi