RX: Make rxi_Alloc return (void *)

RX: Make rxi_Alloc return (void *)

rxi_Alloc returns a pointer to an anonymous data block. Make its
return value (void *) rather than (char *), so that it can be
called in the same way as malloc(), and not require casting.

Change-Id: Ib3094ad0273c62a4430eb56d35e3df68a550d338
Reviewed-on: http://gerrit.openafs.org/2738
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>