Fix includes for BDD_GUARDS

`guard_bddt` needs `bdd_exprt`, but failed to include the necessary
header.

Fixes: #7726

Author: tautschnig

src/analyses/guard_bdd.h

@@ -13,7 +13,7 @@ Author: Romain Brenguier, [email protected]
 #ifndef CPROVER_ANALYSES_GUARD_BDD_H
 #define CPROVER_ANALYSES_GUARD_BDD_H
 
-#include <solvers/bdd/bdd.h> // IWYU pragma: keep
+#include <solvers/prop/bdd_expr.h>
 
 class bdd_exprt;
 class exprt;