rx: Remove rx_atomic_add_and_read trailing semi-colon

Remove a stray trailing semi-colon from the definition of
rx_atomic_add_and_read.

Change-Id: Iae9c62c132bcbb3d2e609dde2d54e0435d36cc03
Reviewed-on: http://gerrit.openafs.org/9075
Reviewed-by: Simon Wilkinson <simonxwilkinson@gmail.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>