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/20231013132832.165768-1-2023002089@link.tyut.edu.cn


Signed-off-by: Hu Haowen <2023002089@link.tyut.edu.cn>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Nicolas Schier <n.schier@avm.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>