scripts: Fix a clerical error in gdb_macros.py

The add operation is meaningless here. New ns should be added to the returned string, so replace + with +=.

Signed-off-by: Luo Yifan <luoyifan_yewu@cmss.chinamobile.com>
Change-Id: I7f0eaaec48d544e21892320dfc1451ebd2e7174e
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/14899
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Konrad Sztyber <konrad.sztyber@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>

scripts/gdb_macros.py

@@ -162,7 +162,7 @@ class NvmfSubsystem(SpdkObject):
         s += '\nnqn %s' % self.get_name()
         s += '\nID %d' % self.get_id()
         for ns in self.get_ns_list():
-            s + '\t%s' % str(ns)
+            s += '\t%s' % str(ns)
         return s