cacheflush.h: Add forward declaration for struct folio

The struct folio is not declared in cacheflush.h so we need to provide
a forward declaration as otherwise users of this header file may get
warnings.

Reported-by: Guenter Roeck <linux@roeck-us.net>
Fixes: 522a0032 ("Add linux/cacheflush.h")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>