kbuild: deb-pkg: Remove blank first line from maint scripts

The blank line causes execve() to fail:

  # strace ./postinst
  execve("./postinst", ...) = -1 ENOEXEC (Exec format error)
  strace: exec: Exec format error
  +++ exited with 1 +++

However running the scripts via shell does work (at least with bash)
because the shell attempts to execute the file as a shell script when
execve() fails.

Fixes: b611daae ("kbuild: deb-pkg: split image and debug objects staging out into functions")
Signed-off-by: Aaron Thompson <dev@aaront.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>