build: fix clang warnings

Change-Id: I3c6a63a18034535f0a8c2c62ba8a708f09d7839b
Signed-off-by: Martin Nowak <dawg@dawgfoto.de>
Reviewed-on: http://openocd.zylin.com/765


Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>