.gitattributes: CR at the end of the line is an error

.gitattributes: CR at the end of the line is an error

When a CR is accidentally added at the end of a C source file in the git
project tree, "git diff --check" doesn't detect it as an error.

    $ echo abQ | tr Q '\015' >>fast-import.c
    $ git diff --check

I think this is because the "whitespace" attribute is set to *.[ch] files
without specifying what kind of errors are caught. It makes git "notice
all types of errors" (as described in the documentation), but I think it
is incorrectly setting cr-at-eol, too, and hides this error.

Signed-off-by: Nanako Shiraishi <nanako3@lavabit.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>