Fix MISRA rule 8.4 in common code

Rule 8.4: A compatible declaration shall be visible when
          an object or function with external linkage is defined.

Change-Id: I26e042cb251a6f9590afa1340fdac73e42f23979
Signed-off-by: Roberto Vargas <roberto.vargas@arm.com>
diff --git a/drivers/io/io_fip.c b/drivers/io/io_fip.c
index a23940d..487f586 100644
--- a/drivers/io/io_fip.c
+++ b/drivers/io/io_fip.c
@@ -69,7 +69,7 @@
 
 
 /* Identify the device type as a virtual driver */
-io_type_t device_type_fip(void)
+static io_type_t device_type_fip(void)
 {
 	return IO_TYPE_FIRMWARE_IMAGE_PACKAGE;
 }