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/plat/arm/board/juno/juno_topology.c b/plat/arm/board/juno/juno_topology.c
index ca5c344..052ab9f 100644
--- a/plat/arm/board/juno/juno_topology.c
+++ b/plat/arm/board/juno/juno_topology.c
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2016-2018, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2016-2019, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -20,7 +20,7 @@
 		.ring_doorbell = &mhu_ring_doorbell,
 };
 
-scmi_channel_plat_info_t *plat_css_get_scmi_info()
+scmi_channel_plat_info_t *plat_css_get_scmi_info(void)
 {
 	return &juno_scmi_plat_info;
 }