gdbserver: fix duplicate declaration

commit da0d1e37 did not merge correctly, causing the build to fail.

Change-Id: I3f525054bb38b7ee29bf27309bb2e6a5bb8329c7
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/2020
Tested-by: jenkins