Make sure all headers do #pragma once. (#6188)

src/smt/database.h

@@ -3,6 +3,7 @@
 Copyright (c) 2015 Microsoft Corporation
 
 --*/
+#pragma once
 
 static char const g_pattern_database[] =
 "(benchmark patterns \n"

src/smt/theory_pb.h

@@ -19,6 +19,7 @@ Notes:
     It performs unit propagation and switches to creating
     sorting circuits if it keeps having to propagate (create new clauses).
 --*/
+#pragma once
 
 #include "smt/smt_theory.h"
 #include "ast/pb_decl_plugin.h"