fmc.h: include linux/io.h to compile on older kernels

kernel/include/linux/fmc.h

@@ -12,6 +12,7 @@
 #include <linux/types.h>
 #include <linux/device.h>
 #include <linux/interrupt.h>
+#include <linux/io.h>
 
 struct fmc_device;
 struct fmc_driver;