scripts/gdb: decode bytestream on dmesg for Python3

The recent fixes to lx-dmesg, now allow the command to print
successfully on Python3, however the python interpreter wraps the bytes
for each line with a b'<text>' marker.

To remove this, we need to decode the line, where .decode() will default
to 'UTF-8'

Link: http://lkml.kernel.org/r/d67ccf93f2479c94cb3399262b9b796e0dbefcf2.1462865983.git.jan.kiszka@siemens.com
Signed-off-by: Kieran Bingham <kieran@bingham.xyz>
Acked-by: Dom Cote <buzdelabuz2@gmail.com>
Tested-by: Dom Cote <buzdelabuz2@gmail.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>