virtio: fix test build after uio.h change

Fixes: d3849953 ("fs: decouple READ and WRITE from the block layer ops")
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

tools/virtio/linux/kernel.h

@@ -23,6 +23,10 @@
 #define PAGE_MASK (~(PAGE_SIZE-1))
 #define PAGE_ALIGN(x) ((x + PAGE_SIZE - 1) & PAGE_MASK)
 
+/* generic data direction definitions */
+#define READ                    0
+#define WRITE                   1
+
 typedef unsigned long long phys_addr_t;
 typedef unsigned long long dma_addr_t;
 typedef size_t __kernel_size_t;