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

tests/sqlite3/usage/BlobSample.h

@@ -1,6 +1,6 @@
+#pragma once
+
 // generated by ../../sqlpp11/scripts/ddl2cpp -fail-on-parse BlobSample.sql BlobSample blob
-#ifndef BLOB_BLOBSAMPLE_H
-#define BLOB_BLOBSAMPLE_H
 
 #include <sqlpp11/char_sequence.h>
 #include <sqlpp11/data_types.h>
@@ -75,4 +75,3 @@ struct BlobSample : sqlpp::table_t<BlobSample, BlobSample_::Id, BlobSample_::Dat
     };
   };
 };
-#endif