Make rxgen emit prototype for ExecuteRequest

Make rxgen emit prototype for ExecuteRequest

Make rxgen provide a prototype in its header file for _ExecuteRequest, and
remove all of the adhoc prototypes from the tree.

Reviewed-on: http://gerrit.openafs.org/247
Tested-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>