cmake-mode.el: Fix indentation of 'closing parens only lines'

Outdent lines containing only closing parentheses (`)`) by `cmake-tab-width`
to align the indentation of the 'closing parens only line' with the line that
contains the opening paren.

I.e. change the formatting from:

  add_library(mylib
    mysrc.c
    )

to:

  add_library(mylib
    mysrc.c
  )