staging: mt7621-mmc: add missing header.

<linux/interrupt.h> is sometimes included by other include files,
and sometimes not, depending on config, particularly CONFIG_HIGHMEM.

So include it explicitly rather than relying on implicit inclusion.
Signed-off-by: NNeilBrown <neil@brown.name>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/mt7621-mmc/sd.c

@@ -38,6 +38,7 @@
 #include <linux/dma-mapping.h>
 #include <linux/spinlock.h>
 #include <linux/platform_device.h>
+#include <linux/interrupt.h>
 
 #include <linux/mmc/host.h>
 #include <linux/mmc/mmc.h>