checkpatch: Ignore __pure $Attribute

Just like "__cold", ignore the __pure gcc attribute macro so pointer warnings aren't generated for uses like "int * __pure fn(...)"

---
 scripts/checkpatch.pl | 1 +
 1 file changed, 1 insertion(+)

index f0bb6d6..b6a850c 100755
@@ -278,6 +278,7 @@ our $Attribute	= qr{
 			__noreturn|
 			__used|
 			__cold|
+			__pure|
 			__noclone|
 			__deprecated|
 			__read_mostly|