Fix compile failure on MacOSX

MacOSX tool chain defines __unused in "sys/cdefs.h", causing a collision.
Remove the local define to avoid polluting the compilers internal 
symbol namespace.

Change-Id: I16370c4518e6aeec482dd689e7db80628f846ee3
Signed-off-by: Matthias Welwarsky <matthias.welwarsky@sysgo.com>
Reviewed-on: http://openocd.zylin.com/4118


Reviewed-by: Steven Stallion <sstallion@gmail.com>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Tested-by: jenkins