rxgen: cast bool properly

The C type for xdr's bool is bool_t. When casting, use the correct type

Change-Id: I562ee1e48eeffa8fece66176cf13013630d157aa
Reviewed-on: http://gerrit.openafs.org/10261
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>