Include missing invariant header

The code would previously compile as irep.h included invariant.h.

Author: tautschnig

src/java_bytecode/java_enum_static_init_unwind_handler.cpp

@@ -11,6 +11,7 @@ Author: Chris Smowton, [email protected]
 
 #include "java_enum_static_init_unwind_handler.h"
 
+#include <util/invariant.h>
 #include <util/suffix.h>
 
 /// Unwind handler that special-cases the clinit (static initializer) functions