If libsdl-native is in ASSUME_PROVIDED, check for it in the sanity tests.
This warns the user if they've said its being provided but it isn't and
prevents silent build issues.
(From OE-Core rev: d9d7b0515fcf47c4cf7533a12915ea92298ce834)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>