scripts/show_delta: add __main__ judgement before main code

When doing Python programming it is a nice convention to insert the if
statement `if __name__ == "__main__":` before any main code that does
actual functionalities to ensure the code will be executed only as a
script rather than as an imported module. Hence attach the missing
judgement to show_delta.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Hu Haowen <[e-mail]>
Reviewed-by: Nick Desaulniers <[e-mail]>
Reviewed-by: Miguel Ojeda <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Cc: Masahiro Yamada <[e-mail]>
Cc: Nicolas Schier <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

scripts/show_delta

···
 	for line in lines:
 		print (convert_line(line, base_time),)
 
-main()
+if __name__ == "__main__":
+	main()