add missing __iomem to generic iounmap declaration

Signed-off-by: Mark Salter <msalter@redhat.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>

include/asm-generic/io.h

@@ -327,7 +327,7 @@ static inline void __iomem *ioremap(phys_addr_t offset, unsigned long size)
 #define ioremap_wc ioremap_nocache
 #endif
 
-static inline void iounmap(void *addr)
+static inline void iounmap(void __iomem *addr)
 {
 }
 #endif /* CONFIG_MMU */