Remove several warnings reported with W=1

Improved support for W=1 compilation flag by solving missing-prototypes
and old-style-definition warnings.

The libraries are compiling with warnings (which turn into errors with
the Werror flag).

Outside of libraries, some warnings cannot be fixed without heavy
structural changes.

Change-Id: I1668cf99123ac4195c2a6a1d48945f7a64c67f16
Signed-off-by: Ambroise Vincent <ambroise.vincent@arm.com>
diff --git a/drivers/io/io_fip.c b/drivers/io/io_fip.c
index eaaf090..544b37d 100644
--- a/drivers/io/io_fip.c
+++ b/drivers/io/io_fip.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2014-2018, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2014-2019, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -48,7 +48,7 @@
 	uintptr_t dev_spec;
 } fip_dev_state_t;
 
-static const uuid_t uuid_null = { {0} };
+static const uuid_t uuid_null;
 /*
  * Only one file can be open across all FIP device
  * as backends like io_memmap don't support