failcmd: make failcmd.sh executable

Change the file permissions of tools/testing/fault-injection/failcmd.sh to
allow execution. This ensures the script can be run directly without
explicitly invoking a shell.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Breno Leitao <[e-mail]>
Reviewed-by: Akinobu Mita <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>