glapi: Correct size of allocated _glapi_table struct

glapi: Correct size of allocated _glapi_table struct

The __glapi_gentable_set_remaining_noop() routine treats the _glapi_struct
as an array of _glapi_get_dispatch_table_size() pointers, so we have to
allocate _glapi_get_dispatch_table_size()*sizeof(void*) bytes rather
than sizeof(struct _glapi_struct) bytes.

Reviewed-by: Jeremy Huddleston <jeremyhu@apple.com>