Include unordered_map where using a std::unordered_map

The code fixed here previously relied on unordered_map being included by
dstring.h, which is only dragged in when USE_DSTRING is set.

Author: tautschnig

src/ansi-c/ansi_c_scope.h

@@ -12,6 +12,8 @@ Author: Daniel Kroening, [email protected]
 
 #include <util/irep.h>
 
+#include <unordered_map>
+
 enum class ansi_c_id_classt
 {
   ANSI_C_UNKNOWN,

src/goto-programs/class_hierarchy.h

@@ -16,6 +16,7 @@ Date: April 2016
 
 #include <iosfwd>
 #include <map>
+#include <unordered_map>
 
 #include <util/graph.h>
 #include <util/namespace.h>

src/java_bytecode/character_refine_preprocess.h

@@ -24,6 +24,8 @@ Date:   March 2017
 #include <util/std_code.h>
 #include <util/mp_arith.h>
 
+#include <unordered_map>
+
 class character_refine_preprocesst:public messaget
 {
 public:

src/solvers/prop/bdd_expr.h

@@ -23,6 +23,8 @@ Author: Michael Tautschnig, [email protected]
 
 #include <solvers/miniBDD/miniBDD.h>
 
+#include <unordered_map>
+
 class namespacet;
 
 /*! \brief TO_BE_DOCUMENTED

src/util/replace_expr.h

@@ -17,6 +17,8 @@ Author: Daniel Kroening, [email protected]
 
 #include "expr.h"
 
+#include <unordered_map>
+
 typedef std::unordered_map<exprt, exprt, irep_hash> replace_mapt;
 
 bool replace_expr(const exprt &what, const exprt &by, exprt &dest);

src/util/replace_symbol.h

@@ -17,6 +17,8 @@ Author: Daniel Kroening, [email protected]
 
 #include "expr.h"
 
+#include <unordered_map>
+
 class replace_symbolt
 {
 public:

src/util/std_types.h

@@ -21,6 +21,8 @@ Author: Daniel Kroening, [email protected]
 #include "invariant.h"
 #include "expr_cast.h"
 
+#include <unordered_map>
+
 class constant_exprt;
 
 /*! \defgroup gr_std_types Conversion to specific types