analyses/loop_analysis.h: fix includes

Direct use of this header file made apparent that it didn't include what
actually is required to use it: the declaration of goto_modelt.

Author: tautschnig

src/analyses/loop_analysis.h

@@ -13,7 +13,7 @@ Author: Diffblue Ltd
 #ifndef CPROVER_ANALYSES_LOOP_ANALYSIS_H
 #define CPROVER_ANALYSES_LOOP_ANALYSIS_H
 
-#include <memory>
+#include <goto-programs/goto_model.h>
 
 template <class T>
 class loop_analysist;