[gnome-db] GdaSqlStatementType must be introspectable

See at Bug #661504, reproduced here:

GdaSqlStatementType is defined at
libgda/sql-parser/gda-sql-statement-struct-parts.h

but that file is not parsed on GIR generation. We have to include
GdaSqlStatementType in order get GdaSqlBuilder bindable through GI.

We can proceed by:

1) Move that definition to a new file and include it from GdaSqlBuider and
GdaSqlStatement

2) Move that definition to other file in the tree.

Do you agree to create a file called:

libgda/sql-parser/gda-sql-statement-struct-decl.h

Move GdaSqlStatementType definition, and potentially any difinition in
parts required to be introspectable?