gdb/testsuite: remove global declarations in gdb.threads/detach-step-over.exp

Before doing further changes to this file, change to use the :: notation
instead of declaring global variables with the `global` keyword.

Change-Id: I72301fd8f4693fea61aac054ba17245a1f4442fb
Approved-By: Andrew Burgess <aburgess@redhat.com>
1 file changed