graphics/shotwell: unbreak after 843c31724a

vala 0.48.23 misorders variable and constant declarations and puts the
definition of DEFAULT_SCALE after it's (only) use:

src/shotwell.p/dialogs/ExportDialog.c:146:43: error: use of undeclared identifier 'EXPORT_DIALOG_DEFAULT_SCALE'
static gint export_dialog_current_scale = EXPORT_DIALOG_DEFAULT_SCALE;

As a workaround, just insert the constant directly.