sparse: don't warn about unknown attributes

sparse: don't warn about unknown attributes

The kernel and other projects use lots of attributes which sparse
doesn't know about. Since these are often used in widely included
header files (e.g. no_sanitize_address or assume_aligned), we get a
lot of noise for no benefit (the compiler proper complains if the
attributes are not properly guarded by suitable version checks).

So turn off these annoying and useless warnings. They can be turned on
explicitly by Wunknown-attribute.

Signed-off-by: Rasmus Villemoes <rv@rasmusvillemoes.dk>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>