glapi: Annotate XML with exec="skip" for unimplemented functions.

glapi: Annotate XML with exec="skip" for unimplemented functions.

Future patches will use this annotation when code generating
_mesa_create_exec_table(), to determine which functions should be
skipped because they aren't implemented by Mesa.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>