[gdb/testsuite] Add missing quote in remote-gdbserver-on-localhost.exp

In a recent commit I forgot to add a double quote before chmod here:
...
      remote_exec build $rsh_cmd chmod go-rx ."
...

Fix it by adding the missing double quote.

gdb/testsuite/boards/remote-gdbserver-on-localhost.exp

@@ -56,7 +56,7 @@ save_vars {rsh_cmd res} {
 	# Make sure build can't see files on remote target.  We can't use
 	# remote_exec target, because we're in the middle of parsing the
 	# target board.
-	remote_exec build $rsh_cmd chmod go-rx ."
+	remote_exec build $rsh_cmd "chmod go-rx ."
     }
 
     # Set remotedir by default, to force remote_download target to give an