[gdb] Fix whitespace in NEWS

Fix this:
...
$ empty=$(git hash-object -t tree /dev/null)
$ git diff-index --check $empty gdb/NEWS
gdb/NEWS:1874: space before tab in indent.
+ 	                          [--basename | --dirname]
...
and add the file to the clean list in
gdb/contrib/check-whitespace-pre-commit.py.

[ I'm not sure if NEWS has an official style.

There are no settings for NEWS in .gitattributes, so the whitespace attribute
defaults to trailing-space (shorthand for blank-at-eol, blank-at-eof) and
space-before-tab.

We could require either spaces only (tab-in-indent) in gdb/.gitattributes:
...
NEWS	whitespace=space-before-tab,tab-in-indent,trailing-space
...
or tab style (indent-with-non-tab):
...
NEWS	whitespace=space-before-tab,indent-with-non-tab,trailing-space
...

For tab-in-indent, we get:
...
$ git diff-index --check $empty gdb/NEWS  | wc -l
228
...
and for indent-with-non-tab instead:
...
$ git diff-index --check $empty gdb/NEWS  | wc -l
40
...
so the more common style seems to be tab style.

But I'm leaving this as is for now. ]