Replace include guards with "#pragma once" in all header files and in the database model generator scripts. (#506)

tests/sqlite3/serialize/compare.h

@@ -1,3 +1,5 @@
+#pragma once
+
 /*
  * Copyright (c) 2013-2015, Roland Bock
  * All rights reserved.
@@ -23,9 +25,6 @@
  * OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-#ifndef SQLPP_TESTS_SQLITE3_COMPARE_H
-#define SQLPP_TESTS_SQLITE3_COMPARE_H
-
 #include <sqlpp11/sqlite3/connection.h>
 
 #include <iostream>
@@ -59,5 +58,3 @@ namespace
     assert_equal(lineNo, result, expected);
   }
 }  // namespace
-
-#endif