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/sgi575/sgi575_plat.c b/plat/arm/board/sgi575/sgi575_plat.c
index d1a1fbf..0d3fd16 100644
--- a/plat/arm/board/sgi575/sgi575_plat.c
+++ b/plat/arm/board/sgi575/sgi575_plat.c
@@ -1,11 +1,13 @@
 /*
- * Copyright (c) 2018, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2018-2019, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
 
 #include <plat/common/platform.h>
 
+#include <sgi_variant.h>
+
 unsigned int plat_arm_sgi_get_platform_id(void)
 {
 	return mmio_read_32(SSC_VERSION) & SSC_VERSION_PART_NUM_MASK;