rx: Export rx_GetCallStatus

rx: Export rx_GetCallStatus

Commit 59d3a8b8 (vos: restore status information to 'vos status')
added the function rx_GetCallStatus to Rx, and used it in the
volserver, but didn't add the function to our .sym and .exp files,
causing a linker error on at least WINNT.

Add the function to the relevant .sym/.exp files, so we can link on
all platforms.

Change-Id: I859ac6d04d8a21eb6f8b4ba3f3720ca318e91334
Reviewed-on: https://gerrit.openafs.org/13820
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>