runtime: fix tab/space inconsistency in runtime-gdb.py

Change-Id: I78c6198eb909e679cf0f776b77dda52211bfd347
Reviewed-on: https://go-review.googlesource.com/45133
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>