fs-perms.txt: Don't tinker with file modes in /usr/src/debug

fs-perms.txt: Don't tinker with file modes in /usr/src/debug

Since 6775feb9fe935ab01fd9cae2b2d3fce5824a9a72 our local "copy" of the
debug sources has in fact been hardlinked to ${S} and potentially other
places too.  This means that any modifications we make to these files
might have wider consequences than intended.

Avoid this potential pitfall by telling fixup_perms() to leave the file
modes in this directory alone.  No great harm will result from shipping
debug sources with a mode other than 0644: if the mode was permissive
enough for us to compile the sources in the first place then it must
also be permissive enough for subsequent debugging.

(From OE-Core rev: 91b02c65d83811738d4c0e4b7c454459430c8b9b)

Signed-off-by: Phil Blundell <philb@gnu.org>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>