vbo: move VBO-private types, prototypes, etc. into new vbo_private.h header

Things which should not be used outside the VBO module.
More public/private clean-ups coming.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>

src/mesa/vbo/vbo_exec_api.c

@@ -46,6 +46,7 @@ USE OR OTHER DEALINGS IN THE SOFTWARE.
 
 #include "vbo_context.h"
 #include "vbo_noop.h"
+#include "vbo_private.h"
 
 
 /** ID/name for immediate-mode VBO */