powerpc/xmon: Remove unused variable in xmon.c

Gets rid of this warning:

arch/powerpc/xmon/xmon.c: In function 'dump_log_buf':
arch/powerpc/xmon/xmon.c:2133: warning: unused variable 'i'

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>