io_dummy: correct sparse warnings

Include io_dummy.h header file.
Use static for device_type_dummy function.
Signed-off-by: Yann Gautier <yann.gautier@st.com>
Signed-off-by: Etienne Carriere <etienne.carriere@st.com>

drivers/io/io_dummy.c

@@ -7,6 +7,7 @@
 #include <assert.h>
 #include <debug.h>
 #include <io_driver.h>
+#include <io_dummy.h>
 #include <io_storage.h>
 #include <string.h>
 
@@ -18,7 +19,7 @@ struct file_state {
 static struct file_state current_file = {0};
 
 /* Identify the device type as dummy */
-io_type_t device_type_dummy(void)
+static io_type_t device_type_dummy(void)
 {
 	return IO_TYPE_DUMMY;
 }