Add missing include

Presumably files that included this previously happened to pull
unordered_set before now.

Author: smowton

src/summaries/summary.h

@@ -16,6 +16,8 @@ It in particular comprises loop, function, and module summaries.
 #ifndef CPROVER_SUMMARIES_SUMMARY_H
 #define CPROVER_SUMMARIES_SUMMARY_H
 
+#include <unordered_set>
+
 #include <goto-programs/goto_model.h>
 #include <pointer-analysis/object_numbering.h>
 #include <taint-analysis/taint_token.h>