asm-generic/io.h: give stub iounmap() on !MMU same prototype as elsewhere

It made -Werror sad.

Signed-off-by: Adam Borowski <kilobyte@angband.pl>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

include/asm-generic/io.h

@@ -957,7 +957,7 @@ static inline void __iomem *ioremap(phys_addr_t offset, size_t size)
 
 #ifndef iounmap
 #define iounmap iounmap
-static inline void iounmap(void __iomem *addr)
+static inline void iounmap(volatile void __iomem *addr)
 {
 }
 #endif