[orc-rt] #include <variant> for std::monostate. (Fixes unit test on Linux)

Author: lhames

orc-rt/include/orc-rt/IntervalSet.h

@@ -15,6 +15,8 @@
 
 #include "IntervalMap.h"
 
+#include <variant> // For std::monostate.
+
 namespace orc_rt {
 
 /// Implements a coalescing interval set.